Sorcerers have the fighting capability and saving throws of magic-users (save that they gain +3 to save vs. any kind of magic), but turn or control undead as clerics and have the hit dice of clerics.
The magic system is as laid out in Chainmail, using Spell Complexity, with slight modifications.
1. The Complexity of any given spell is its level. An interpretation of the system: Chainmail gives 3 values (Negate, Delay, Immediate) for 2d6 roll results. I've always assumed these as ranges. Thus for a Seer casting a level 1 spell, 5 or below fails, 6-7 is a delayed effect, and 8+ is immediate. An easier way to list this would've been "6/8" for Delayed/Immediate target numbers, but who is to figure out the intricacies of Gygax and Perrin's minds?
2. Sorcerers are still restricted as to the maximum level spell they can cast.
3. Level titles depend upon whether the sorcerer is of an arcane bent (a secret sorcerous cabal, or a sole arcane practitioner) or of a Priestly order, and are as follows:
- Red Wizard
- Black Wizard
- Minister (Treat as Magician for casting)
- Deacon (Treat as Warlock for casting)
- Brother Priest (Treat as Sorcerer for casting)
- High Priest (Treat as Wizard for casting)
Wizards advance as casters according to their level title (i.e. Seer, Magician, Warlock, Sorcerer, Wizard).
4. If a Sorcerer rolls a natural "2" on his spellcasting roll, he suffers backlash--1d6 points of temporary/subdual damage, and loses access to that spell for 24 hours.
5. Corruption: Magic is alien and inherently corrupting. Whenever a wizard casts a spell (successfully or unsuccessfully) he must make a standard saving throw against spells. If he fails, he suffers a cumulative -1 to all future saves and becomes a bit less human. At first this begins with his outlook--he slides ever more towards Evil/Chaos. Every two saves failed slide him one rank closer to evil. Once he is evil, he begins suffering physical effects. Exactly what these effects are is up to the DM, but they should include things like glowing red eyes, jet black or scaly skin, an aura of corruption that makes those around him uncomfortable, causes plants to whither, food to rot, etc., vestigial tentacles, clawed hands, gaunt appearance...the options are nearly endless.
6. Sorcerers must still prepare spells every morning per the Magic-User "Spells per day" table on Men & Magic p. 17, but do not forget spells when cast unless they fail a casting check.
Spell Lists are a combination of Magic-User and Clerical lists, save the following spells which are not available as they don't fit the flavor of the class:
Fire Ball (3rd MU), Invisibility 10' r (3rd MU), Wall of Fire (3rd MU), Wall of Ice (3rd MU), Reincarnation (6th MU).
Where a spell appears on the spell lists for both classes, use the HIGHER level as default. I.e. if a spell is available to magic users at level 2 and clerics at level 3, it is available to sorcerers at level 3.
Generally, very "flashy" or explosive direct-damage spells are unavailable. Direct damage is generally limited to things like death spell, cloudkill, disintegrate, etc. Fireball is too much of an "artillery" type spell, though Lightning Bolt has precedent in the "evil sorcerer" mold; however, Lightning Bolt should be restricted to a single target rather than everything in a line. DMs should keep this in mind when allowing or disallowing spells from later sources.
Due to their combined spell list, increased hit dice, ability to re-cast spells and ability to turn/command undead, Sorcerers have a different level progression than Magic Users:
1 - 0
2 - 3000
3 - 6000
4 - 11000
5 - 25000
6 - 50000
7 - 75000
8 - 100000
9 - 150000
10 - 200000
11 - 300000
12 - 400000
13 - 500000
14 - 600000