Generally you make up an ability for a monster and assign a point cost using the Aberrance pool that monsters generally possess.
Aside from that, just asign General skills that fit the character and remember that in general a monster refreshes if it isn't in scene for at least one scene, with the possible exception of health.
Assume the average attack from a weapon will do ~3 damage (less armor), guess how many weapon attacks should kill the monster (if it can be killed) and multiply the values to get its health pool.
Scuffle ratings are likely to be high with most monsters and they'll probably get a damage bonus of some kind.
Consider any special abilities you want and give them an Aberrance cost, usually between 1-3, and assign the monster an Aberrance rating based on how horrible, inhuman, or unknowable it is.