So you want to make your own monster classes? Here's the general guidelines I use for myself.
Monster classes building guidelines.
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 and size 4. Put the abilitiy descriptions by order they're gained and their names italized, the monster HD, skills and class features should be bolded.
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. 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.
2-If it's not an undead or construct, then ignore the basic monster saves, skills, proefeciencies and Bab progression. The monster class will have those based on the extra abilities they have. The more and/or better abilities, the worst saves/skills/Bab/proefeciencies the monster should have.
2a-If it's an undead or construct, no good fort or ref saves, 2+int skill points per level and no class skills. This is for balancing the powerfull traits of those creature types. In adition, constructs get average Bab and bad will saves and undeads get bad Bab and good will saves. That's because that it's easier to buff an undead but there's also several anti-undead spells that target will, and vice-versa.
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-For Nat armor, base it on Con modifier. The default is Nat armor=Con modifier. Stronger monsters can have something like X+Con modifier (keep X low tough), or half Con modifier for "softer" monsters. 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, or replace it by a deflection/insight bonus based on a mental score.
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 very really good reason.
5a-Speaking of wich, 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 but has no actual Int-based abilities it's preferable to simply give it skill points than Int boosts.
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), or yet only can rely on basic attacks to damage oponents.
6-Break down the monster abilities into separate groups, figure out the levels on wich it would be fair for them to be available to a player, then distribute them trough the monster levels so you gain something nice at every one.
6a-Flying should only come at 4th level, unless you're an handless monster whitout ranged attacks whatsoever. Growth can come online as early at level 3, but it's preferable at 4th level or higher.
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 become available at higher level), and SR should be 11+HD, or 15+HD for monsters that are really suposed to be anti-casters like the Rakasha.
7-Make sure that at least half the monster abilities scale with HD. SLAs saves should always be 10+1/2 HD stat and the monster should gain more uses when it gains more HD. Yes, HD, not monster levels.
7a-Try to put sinergies with other classes, like the ogre mage's ability to gain extra caster level and higher level spells when it. 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 put synergies with classes not available on the srd.
7b-At-will SLAs just in special cases. Even tough most monsters have them, at-will is considerably stronger on the hands of a player that will live for more than 5 rounds. Please don't make every single SLA of the monster at-will. You can put a couple effects or iconic SLAs, but think carefully, does the player really needs them? Wouldn't it be best to give some custom ability instead?
The exception to the above rule it's for epic play. Monsters who reach lv 21 or higher can have all their SLAs at-will as the power level is considerably increased at that point.
7c-It's however ok to allow certain SLAs to 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, and Summon monster X becoming X+1 every other level. This should only be used for SLAs that become considerably weaker after a certain level. Dispel magic for example is caped at +10 CL so it stops growing after you reach 10 HD.
8-Compare with base classes. I'm trying to aim at around sorceror-warblade power level around here. If you see anything that you think would curbstomp a wizard, please slap me.
8a-Compare with other monsters here of the same type/role. Gloom is the base "roguish" monster for example.
9-If a certain monster has several versions (like the diferent age categories of each dragon), then it's ok to combine them all on the same class, but try that abilities of each "stage" of the monster are gained at the apropriate level/CR. For example, the dragon classes grow at the levels corresponding at the CRs the. On the other hand, the dragons get their best SLAs pre-epic because they wouldn't be much usefull if only gained at epic levels.
10-Faster than linear progression. Each levels 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 as the prime example 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.
10b-On the same vein, monsters with really powerfull abilities should receive custom penalties, 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 untill it reaces full power.
11-Sizes smaller than small or bigger than medium offer too many advantages (despite their penalties) for first level. Because of that monsters either start medium or small, and then they grow up to their full size over the levels. Since it makes considerably less sense to shrink, starting as tiny or smaller is possible with a custom penalty like the Imp.
12-Make sure your monster class can be played with just core books and material easily available on the web like the srd and public wotc articles. If you want to include non-core spells/feats or similar, make instead custom abilities. You may still include splatbook material as alternate class features.
