You have to be able to translate with your hands what you hear in your head. In order to play a fill, first you have to have an idea for the fill. You have to think of something that's appropriate to the song style. (You probably wouldn't put a double bass metal fill in a rockabilly song for instance) Then hopefully, you can faithfully reproduce what you hear in your head with your limbs. To me a fill is something you feel, based on the musical color you're painting, not a pre determined combination per se. In order to master the drums you have to be able to execute anything on your instrument without thinking about it, so you have enough brainpower left over to listen to what the song is asking for and be able to think of appropriate rhythmic ideas for fills, transitions, endings, dynamics etc. all while playing your instrument. Your hands and feet are the wheels to get you there and your mind is the navigator, planning the trip. A fill can be thought of as a rhythmic transition. You have to pick a rhythm that soothes the song. Listen. What does the song require? Practicing combinations is good, in order to be able to execute them smoothly, but they are only the rhythmic "syllables", not in context. It's all in how you arrange the "syllables" that make a musical statement. You need a good fill idea based on the mood of the passage coupled with deft dynamic execution. You can't teach someone how to feel things, either you have it or you don't.