CSSS is great for its vast learning curve, starting as nobodies and becoming superheroes.

Whatever you do, dont use NWC. Rly, its a cheat, it makes game too easy. Starting with low stats and low gold is a nice thing, it adds depth to starting choices.

While distributing stats get as low Luck as possible, you will have a well to up that in New Sorpigal.
Speed is of very little use to casters. You can get AC from items. Speed does not hasten your spellcasting.
Might is of very little use to casters. At start you cant afford to swing in melee, you die in three hits, and later you get stat boost that means your high starting might would only give you +1 dmg most.
Accuracy can actually help hit with bows more ofter at start, if you plan to use them alot until your magic becomes powerful.
I would take every character's mana defining stat to 21 or 25, try to get high Endurance as well.

Now, there are two ways to do it.
Hard one: start at New Sorpigal and complete everything there is (clear out the main island of monsters, do the two dungeons) without leaving to Castle Ironfist. I love to do it because, honestly, bows are kinda imbalanced in the early game.
Easy one: start by taking coach to the Castle and getting extra 5000g, bow skill and bows for everybody, cure wounds spell for cleric (dont forget 1000g in New Sorpigal tavern =).

Without bows, you will have to either sleep alot in dungeons or backtrack to the temple, but i find fun in it. I used static charges and water bolts to substitute bows and that was feeling more "magic" Cleric can shoot with harm or mindblast too.

First thing i would do is buy every reagent and potion bottle i can. They are VERY useful to mix black potions for primary stats ASAP. You'd love that bonus health points, mana points, whatever you fit. Dont mix everything - its sometimes better to stay at 15 to one attribute than 10 to both.

Keep Wizard Eye up at every moment. Simple as that.

Then i would not sell or buy anything until i get a chance to buy merchant skill (At the evening in the thieves guild).
I would also look for two NPCs - two +Exp ones (bonuses stack if you get two differnt NPCs - for example, the one that gives small bonus and one that gives big bonus, or the one with big bonus and the one with big bonus but reputation penalty. Reputation is not important unless you are trying to Master Light magic but that happens later in the game, right now, you can afford "minus reputation"). Another option is one NPC with +Exp and other with +Trading. This yeilds more gold. I mostly go this way, but experience is good too!
And get Learning for every character asap as it gives 10% exp bonus just for getting the first level.

I would plan my character development in advance.
I prefer to have those professions mastered by one member in a party (later trading for everyone):
Repair Items
Identify Items
Merchant
Disarm Trap
Perception

So i would buy them when the dusk sets in New Sorpigal for the first time, i would mostly buy Repair and Identify for one character and Disarm and Perception for another, so the first is the one who "looks" at items and other is the one who loots items. Then every character gets Merchant (training discounts), one will get it to master asap, and every character gets Learning (get it to expert when i have expert air magic on everybody, and expert in their professions). And the fourth person will put his points into master of water magic (to have a good lloyd's beacon and enchant)

With magic, you have alot of choices, but mostly there are several "overpowered" spells in the game which are the basis, others are just for having fun You can have fun whichever way you want, but here is how you OWN the game

In the order of acquiring

1) Sparks. This is the best single target spell in the game, considering mana per damage. In dungeons it just rocks, since sparks bounce off walls. In the open you have to go melee and cast sparks enough times so your opponent dies before he can finish his swing on you (and hit you). Every character should get sparks ASAP (first spell) and get air expert as their first skill (to up sparks quantity). You should consider doing mage promotions asap too so you can get master air magic (on lvl four of skill!) to have even better sparks and usable flight.

2) Fireball. This is usabale as a substitute for outdoor area-of-effect spells before you can afford them (and acquire them). If you put points into fire magic to expert, this will already do 4x damage of lvl1, and if you cluster enemies, it can deal very much damage for low mana. You really need to get those enemies together though which can be problematic. Good for taking out those slimes as well.

3) Jump. This is required in some paces, can save your life sometimes or just speed up the dungeon/outdoor route. Should buy this when you can.

4) Enchant Item for one character. This makes money of every unenchanted item you loot, and later lets you just make money buy buying items at shops, enchanting them and selling back. Items will break in process so you need a skilled Repair guy to repair them. You can enchant items in other character's inventory by pressing 1-4 when you used "enchant" spell and are ready to select and item to switch between characters, use that to enchant items in Repairer's inventory so you dont have to give them over to fix them if they break.

4) Water walk and later Flight for at least one character. First makes some maps easier and lets you access hidden islands, second makes game just tenfold easier! Getting flight is the most gamebreaking thing you will ever do. Its so powerfull, beyond your imagination. Some people even consider NOT using it. First, you move just twice as fast with it. Second, every outdoor monster is just a weakling now, since he just cant hit you (you can still raise/descend in turn based to dodge shots, and in real time you just fly too fast). You can rampage every outdoor location with just flight and arrows, you can kill dragons on level one (yet that will take time...), whatever you like... possibilities are endless! Enjoy

Also somewhere around here you should buy Town portal and Lloyd's beacon at least for your water to-be master. It saves enormous amount of travel time and it lets you set a beacon at a temple, then in dungeon set another beacon, recall to temple, heal and restore spell points, and get back to the dungeon. Saves time and can help in very hard situations, where you are pinned in a corner and would otherwise die. Later you would buy beacond and portal for everybody, so your master would be the one who sets beacons in areas where portal cant take you, and others do the dungeon porting job.

5) Starfall or Meteorite shower, one of the two for every sorcerer, later both. After you get this - its pretty much over for the outdoor monsters. They are all a bunch of XP and money and 0 threat. You can just walk in range and cast it and it will kill them instantly because of the insane damage output (each star hits alot of monsters, and each monster is hit with almost all stars). To use it at its full power, you can either:
a) attack monster from beyond maximum range. for that, ready the spell as a quick spell, then in real time when you just see the monsters but they are not yet showing on your minimap nor are "active", shift-click one with your mouse and the spell will fire over their heads, they cant even retaliate!
b) stack monsters by flying circles over them at high altitude, this will gather them in a tight pack, then fly straight away from them, enter turn based, turn around, and kill them all in one-two casts. You will notice game has a limit on how many monsters can "act" in realtime mode, others will stay still, so it might take two tries. To stack ranged monsters, get over them at high atitude, then repeatedy enter time based, exit time based as soon as you are given opportunity (every monster who decided to did a shooting attack), move a bit (so arrows flying towards you dont hit you), enter time based... each time you enter time based, every monster (ranged or not) will walk towards you, so you can make them bunch up even if they normally just strafe around.
What is even more overpowered is that this spell does not require any skill to be already devastating. Even lvl1 air magic 's Starfall is a destruction. Expert 4 makes it even better, and since you can get master at level 4, you are insanely overpowered already hours(of real life time) into the game!

6) Ring of Fire. You can get this earlier but its highly situational use so you might skip it to hold on your money. This spell is very powerful because it Clips through walls! It is insanely overpowered in werevolves lair, where you confront just huge amount for very beefy creeps that knock you out in one hit no matter how high health pool you have. It is also useful everywhere where monsters are close to a wall/door you can stand on the other side of and safely dispatch them. Does litte damage per high mana use so might still skip it.

Dark magic comes in

7) Shrapmetal. This is the most powerful spell in the game in terms of dealing damage per second. You can kill a dragon (most powerful monster in the game) in a melee in less than one second, before it has time to claw or bite you. Bam! The spell is not most effective on mana usage, but the damage is insanely huge. Requires points and dark magic mastery though, but mastery is easy to acquire since you just need to kill some people to become Notorious This spell is used to kill dragons, wyrms, and to kill everything that must be killed fast, where sparks wont cut it.

8) Day of Protection, Day of the Gods, Hour of Power. Theese spells (light and dark magic) are insanely overpowred as well and make alot of other spells useless as they cast them all at once. More then that, their effect is multiplied, so your cleric or sorcerer will never be able to keep up in magnitude. For example, Master of magic with a skill of 10 casts theese individual spells as if he would be a Master of said (spirit/body/air/..) magic with a skill of FOURTY! this is just insane. Avoid Hour of Power when you dont need it though, as it will cast Haste, and Haste when it ends makes your characters Weak which is a nuisance and pain in the ass (Especially with 1 cleric).

9) Armageddon
If you didnt have enough fun with starburst and meteorite shower, you can have even more fun now. Armageddon damages every monster on the map. Even if you've never discovered him. Catch is, the spell is supposed to be limited to "x uses per day" but you can actually skip time (not rest) to be able to cast it again, while monsters wouldnt have chance to regenerate. So you can wipe all monsters, no matter how many health they have, if they are not immune to Armageddon that is, without moving a feet! I rarely used it though, as its more fun to starburst them

Your Cleric will only need Heal(Cure?) Wounds, Power Cure and Shared Life from his three schools. Later you might want to get all those "cure X" spells for cleric and sorcerers (stone to flesh is in earth school, for example). Buffing spells are sadly worthless since you get so powerfull "all at once" buffs in Light and Dark schools.