0-Put your monster class into a post of his own, statistics spoilered, spoilered comments below and a non-spoilered picture above everything, monster name in bold. Put the ability descriptions by order they're gained and their names
italized, the monster HD, skills and class features should be
bolded. Use other monsters as guidelines.
1-Number of levels of the class=CR of the monster (or CR increase on case of template-classes). This is as simple as it gets. If the monster is epic CR then you can cut it down to 20 levels,
though it's still okay to go into epic if you'd prefer- in that case, though, it's generally a good idea to give all of its important abilities by pre-epic. If the monster is less than CR 20 and you feel inspired then you can do some extra levels for some kind of "paragon" or "lord" monster with completely custom abilities-
include these in a different statblock.
2-If it's not an undead or construct, then ignore the basic monster saves, skills, proficiencies and BAB progression.
Use your own judgement to determine these- a huge, hulking outsider shouldn't have good reflex saves, etc. The more and/or better abilities, the worse saves/skills/BAB/proficiencies the monster should have.
As for BAB: if a monster is very strong but not skilled, it should have a strength bonus every level and 3/4 BAB. If it is skilled but not strong, it should have more scant strength bonuses and full BAB. However, if it feels lacking, it's always okay to give both full strength bonuses & full BAB. After all, melee needs nice things too.
2a-If it's an undead or construct, no good fort or reflex saves, 2+int skill points per level and no class skills.
Generally- exceptions are possible, but you have to have a really good reason. This is for balancing the powerful traits of those creature types. Constructs get average BAB and bad will saves and undead get bad BAB and good will saves.
Ditto.
3-Monster classes remove any base race advantages at first level. Template classes do not. Take in acount this so 1st level of the monster class can be better than 1st level of most classes because you're sacrificing your race, roughly worth two-three feats.
4-Base natural armor on Con modifier. The default is Nat armor=Con modifier. Stronger monsters can have something like X+Con modifier (keep X low though), or half Con modifier. Undeads and constructs should base it on Str. For "smart" or "spellcaster" monsters you may make the Nat armor based on a mental score instead,
but if it is fullcasting, don't use its spellcasting ability for natural armor.
5-Ability score increases shouldn't be more than +1 to any score in any level. You may take exceptions to this
at 1st level in certain cases. No ability score penalties unless you have a really good reason.
5a-Speaking of which, try to base each monster class in two stats, three at max. Don't go around giving bonus to every one of the six stats, players really don't need it and it clogs up the class. If you feel like a monster should be smart, it's preferable to simply give it more skill points than Int boosts.
Ability score bonuses should directly relate to the class- if the class doesn't use intelligence (other than for skill points), don't give it an intelligence bonus. If it doesn't use charisma, don't give it a charisma bonus. Etc.
Generally, constitution is always okay to give, as is strength for a melee class or dexterity if it's applicable.
5b-Extra scores should only be used if the monster is gaining minor abilities or has bad base statistics (low Bab, saves, no proefeciencies, no hands).
6-Break down the monster abilities into separate groups, figure out the levels on which it would be fair for them to be available to a player, then distribute them through the monster levels so you gain something nice at every one.
No dead levels, no levels where you only get ability scores. Period.
6a-Flying should only come at 4th level, unless you're a handless monster without ranged attacks,
or anything crippling along those lines. Growth can come online as early at level 3, but it's preferable at 4th level or higher.
See Hyruda's Wyvern's Leathery Wings ability if you feel you need some sort of flight before 4th level.
6b-Melees deserve nice things. Casters deserve bad things. DR should be equal to half your HD (except for really tough monsters like the iron golem, and even then it should be 1/2 HD initially, equal to HD later). SR should be 11+HD, or 10+HD+Ability score for monsters that are really supposed to be anti-casters like the Rakasha.
If you feel the need for fullcasting on a monster, pare down the abilities. A lot. Generally, casting equal to the class levels minus one is the rule, but otherwise is certainly possible.
7-Make sure that most of the monster abilities scale with HD. DCs should always be 10+1/2 HD+stat (usually charisma or constitution) and the monster should gain more uses when it gains more HD. Yes, HD, not monster levels.
7a-Try to put in synergies with other classes, like the ogre mage's ability to gain extra caster level and higher level spells when it multiclasses. And again make several (but not all) of the abilities scale with HD and not necessarily monster class level (except for the strongest abilities).
Do not, however, use synergies with classes unavailable on the srd.
7b- Use At-Will SLAs very, very cautiously. Generally, the only times they should be used are if it is vital to the class, and not spammable.
7c-SLAs should sometimes auto-upgrade to stronger versions when the player reaches a certain HD. The prime example of this is Dispel Magic upgrading to Greater dispel magic at 11 HD, Summon monster X becoming X+1 every other level,
or teleport becoming greater teleport. This should only be used for SLAs that become considerably weaker after a certain level. Dispel magic for example is capped at +10 CL so it stops growing after you reach 10 HD.
8-Compare with base classes.
If it is as powerful as a rogue or barbarian or ranger, kick it up a notch: if it's as powerful as a druid, you've gone a little too far.
Generally, the dragons are a good balance point, while the tarrasque, anthropomorphic animal or ghaele are as powerful as you should get.
8a-Compare with other monsters here of the same type/role. Naztharune Rakshasa is the base "roguish" monster for example.
9-If a certain monster has several versions (like the different age categories of each dragon), then it's ok to combine them all on the same class, but abilities of each "stage" of the monster should be gained at the apropriate level/CR. For example, the dragon classes grow at the levels corresponding at the CRs the actual monsters. On the other hand, the dragons get their best SLAs earlier because they wouldn't be much usefull if only gained at epic levels.
10-Faster than linear progression/low quadratic. Each level should give more than the last one, even if just a little.
10a-Don't be afraid to make custom abilities if the original monster doesn't have enough to fill all levels (check out the giants,
harpy, or iron golem as prime examples of this). Keep them creative, in theme for the monster and fun, not necessarily powerfull. One simple idea is to allow the monster to partially ignore immunity to his main combat tactic.
Also, granting choices such as the iron golem's upgrades or the harpy's song is always nice.
10b-On the same vein, monsters with really powerfull abilities should receive custom penalties
/reductions, that are reduced as the character grows. The ghost, being undead and ethereal must carry his body around. The vampire must drink blood to fuel his powers. The Medusa's gaze starts weak and then improves until it reaches full power.