I take a piece of fleece about 1/4 inch thick - or less.
I softly roll it around the needle then flatten it by stabbing it a little onto the foam pad.
If you havent been felting long just make the lower jaw totally on the foam pad turning it over often.
I prefer to be more hands on at this point, but there is a risk of stabbing your fingers if you loose your concentration.
I hold the lower jaw between my finger and thumb
and gently stab into it to smooth the edges and firm up the whole jaw by taking the loose fibers from the edges right down into the middle of the jaw. I leave the end a little fluffy to make it easier to attach to the head
Using the loose fibers I then attach the lower jaw to the head adding more fleece if needed and always looking at the face from all angles to make sure it is looking right. Remember to open the mouth and felt inside as well as outside
At this stage I always think they look a little like a duck or a donkey, I think this guy looks a little mole like too :D but if the basic shape looks OK then once you add the other features it will begin to look more like a dog soon!
At this point spend a long time (at least an hour) making sure the head is really firm and smooth, keep looking at it from all angles and add more fleece to any areas you need to flatten out