As the guy who wrote up those class breakdowns, here's what I'd do.
1. Figure out its ECL
2. Spread its HD over that many levels
3. Spread its special abilities (including stat bonuses) over that many levels.
3a. Keep in mind that flight shouldn't be available for most creatures until about 5th level (when a wiz would get the fly spell).
3b. Keep in mind that things like DR are too powerful for low levels.
3c. Reach of 10 or more shouldn't be available until about 4th level because it's a strong ability, especially at lower levels when you're often fighting multiple weak creatures (who have to swarm you, and thus you have many opportunities for AOOs).
3d. Don't award a spell-like ability until the ECL of a similar class that could get that ability as a spell (so don't give out invisibility until about level 3, fireball until about level 5, and so on).
4. Spread its feats and skill points over that many levels. A creature with feats should get its first feat at level 1, like normal characters.
5. Always keep in mind the power level of the monster compared to a suitable character at that ECL.
Good luck!