Monster classes are a breakdown of creatures' benefits into levels and presented as advancement tables. These are for creatures whose minimum ECL as presented in their creature description is above 1st, so that a player may be allowed to begin play at 1st level as her race of choice.
Monster classes should always start with a racial HD or—if the base creature is 1-HD creature—a class level.
Monster classes should start with all the race's ability score decreases.
Monster classes should start with the total ability score increases and decreases equal to +2/ +2/−2 (if possible and reasonable).
Monster classes should spread out the rest of the ability score increases evenly throughout the advancement.
Adjustment levels and racial HD increases should be evenly spaced out throughout the advancement.
Monster classes should spread out the racial abilities throughout the advancement, giving the weaker abilities in the earlier levels and the stronger abilities in the later levels.

BAB (Attack Bonus Progression)
1:1 Fighters (1-2-3-4-5...)
3/4 Rogues, Bards , Rangers, Clerics (0-1-2-3-3...)
1/2  Arcane (0-1-1-2-2...)

HD
d6 , d8, d10, d12

Saves
Good save progression: +2, +3, +3, +4, +4, +5, +5, +6, +6, +7, +7, +8, +8, +9, +9, +10, +10, +11
Poor save progession:+0, +0, +1, +1, +1, +2, +2, +2, +3, +3, +3, +4, +4, +4, +5, +5, +5, +6, +6, +6, +7

If Strong = good Fort Save
If Fast = good Reflex save
If smart/wise = good Will save

Example Classes

Bard F=Poor, R=Good, Will=Good
Cleric F=Good, R=Poor, Will=Good
Fighter F=Good, R=Poor, Will=Poor
Rogue F=Poor, R=Good, Will=Poor
Sorcerer/ Wizards F=Poor, R=Poor, Will=Good