dont know if this helps... but with me i always 1- drum on the desk, u know when you at school and someone is rapping you beat on something... so i learned how to play a metronome if i havent started a track already... but anyway ill start something at a tempo.. and try to improvise and drum on the desk how i want my snare roll to go.. and then lay it down..
thats pretty much the hard part im use to it, but when you get in the piano roll for the 1st time trying to do a snare roll the way you hear it, its kinda difficult
or you can just go to the piano roll and just click stuff see what it sound like... and then arrange accordingly.. a trial and error type thing bro