More specifics would help but I would suggest getting an anatomy diagram of the human musculature and putting snakes or rolls on top of your base figure where major muscle groups are and smoothing it out. Build it up slowly. You can always sand in the bulking stage if something seems disproportionate. When you get to detailing it will be easier if the basic shape is already in place. Make sure you have a detailing tool. If you feel at all uncomfortable, practice on some sculpey or oil clay before wasting air hardening sculpt. An understanding of the underlying muscles really helps though. Any areas that are small but bulk up, you can roll the sculpt into smaller balls once the other layers are dry and sanded. Then smooth them into the area.