advice adding class levels to monsters

[Malfrick]

I am trying out adding levels to monsters that are not in the advanced race guide, such as a green hag or such and I have not yet found the guidelines or "rules" for doing this in any of the books.

Can anyone direct me towards a spot in the books or the PDR?

Thanks

[Jeraa]

The back of the Bestiary has rules.

It can also be found here in the PRD. Just scroll down to "Adding Class Levels".