Here's some exercises to do to help you develop you foot into fills. Start slow and work your way up.
s=snare, t=tom-tom, B=kick or bass drum
stbb stbb stbb stbb, so you are going: ( snare tom kick kick) it should sound like badoboomboom badoboomboom badoboomboom badoboomboom
bbt bbs bbt bbs bbt bbs bbt bbs (kick kick tom, kick kick snare) boomboomdo boomboomda boomboomdo boomboomda
btbb sbss bsbb tbtt btbb sbss bsbb tbtt (kick tom kick kick, snare kick snare snare, kick snare kick kick, tom kick tom tom) boomdoboomboom daboomdada boomdaboomboom doboomdodo