If you have the ability, take a private lesson with a gymnastics coach. A back extension roll is a pretty fundamental gymnastics skill and is in the early compulsory routines (I believe it's in L4 and maybe L5, but I get mixed up now that they changed all of the levels a few years ago). Anyway, there are some tricks, like pushing your butt back and snapping while you roll, that will make the skill easier and much prettier. You want your fingers turned in and touching and you want your arms straight, then you snap and roll right onto your hands. You shouldn't be bending your arms or pushing to a handstand at all. Also, lots and lots and lots of practice on the cheese mat. Good luck!