Move your cursor to the "Packs" menu on the left hand side of the screen. Scroll through this menu until you find the drum you want to use. Snares and toms work well in drum rolls. Drag the drum into the track menu, right click it and select "Piano Roll".
Click beside the piano key on the piano roll to choose the pitch of the drum sound. This will create a green note marker, drag this note marker so it covers four squares in a horizontal row on the piano roll.
Right click on the green marker and copy it, pasting it so that it follows on from the first note horizontally but is in a different position vertically. The vertical position of the new note depends on whether you want the drums to roll up or down. Repeat this once more, leaving you with a sequence of three evenly spaced drum notes.
Select the "Light-through" tool in the toolbar just above the piano roll and drag the tool over the first note to select it. Select the "Tools" menu on the toolbar just above the piano roll and select "Chop" to bring up the chop menu. Use the "Time Mul" dial in the chop menu to alter the length of each chop.
Repeat step four for each of the notes, keeping the chop lengths the same. Press play to preview this and it will give you a rapid drum sound. Click on random chopped sections of each note on the piano roll to delete them. Removing three or four chopped sections from each note will make it sound more like a natural snare roll.
Alter the beats per minute -- or bpm -- rate in the main toolbar of FL until you find a pace that you are happy with. A natural sounding pace will be between 75 and 95 bpm.
Click on the Velocity tab in the tool bar above the piano roll. Select Note Panning from the drop down menu. This will change the field under the piano roll to the Stereo Field. You can then drag the notes into different positions in the stereo field to give different panning effects. Putting notes at the beginning of the roll high in the stereo field, gradually getting lower as the roll progresses will cause the roll to pan from left to right in stereo speakers.