Playlist clips in FL Studio can be sliced up and rearranged, which is an easy way to add fills and syncopated variations to drums. Once we have some variations, we can merge the clip slices and then clean it up a bit in the piano roll.
Make a simple drum beat in an empty pattern. I'm going to use FPC and sequence the beat in the piano roll, but using the step sequencer will also work. One or two bars will be enough.
When you have a beat that you're happy with, go into the playlist and paint a few pattern clips with it. I'm going to make the beat repeat for four bars, but add a bit of variation to the second bar and a little fill at the end.
Grab the slicer tool in the playlist by clicking the slicer button or pressing 'C'. Think about how you want to rearrange the beat, or just slice the clip into beats or 8th notes. With the slice tool, click and drag across the clip to slice it, or hold shift and click if you don't like dragging. I'm going to have the second bass and snare drum hits one beat earlier and move the ghost snares to the end of the bar, so I sliced the clip at the seventh and eleventh 'step's.
If the beat has hits that start a little early, you might notice that they won't slice as nicely. Here, I've moved one hihat back a few ticks, and while it was dead on the slice point before and was included in the sliced clip, now it's a part of the clip that's been left behind. To correct this, hold alt to temporarily disable snapping and resize both clips with the slip tool, 'S', so that the hit appears in the sliced clip and disappears from the other.
Now we can start rearranging the beat. Each sliced clip can be moved wherever you like. Putting the slices on tracks below the main beat might help you figure out what you're doing. After I rearranged my clips, I didn't really like how it sounded, so I just sliced up the last beat in half and put one slice ahead of the second bass and snare drum slice. This is all non-destructive, completely undoable editing, so feel free to try out a lot of stuff. If you really mess up badly, you can just delete all the slices, paint the pattern clip in again and start over.
Once you're very happy with your rearranged slices, it's easy to 'merge' the slices back into one clip, so you don't have a bunch of slices floating around everywhere. Just select all the slices, go into the main playlist menu and from 'Edit', select 'Merge pattern clips'. Now all the slices are glued together in a completely new pattern. This is ideal if you want to go back in and add some fine-tuned variations in the piano roll, and makes everything a bit neater.
Each slice can be placed very close to another one, so crazy rolls, buzzes and flams are possible. This is what I did for the fill at the end of my drum phrase, by putting slices with the snare drum overtop of each other.
Once you've merged the clips together, it's a good idea to go back and edit the drum pattern a bit, so that the edits are a little more natural. Changing velocities and slightly adjusting the groove will make it all flow together nicely. I'm going to make my little fill at the end less machine-gun sounding and move some of the accents of the hihats around in both merged clips.