In my latest book, The Teacher and Student Method Drum Set Studies Exercise Book One, I wanted to simplify the process for a beginning drummer learning how to create their own fills. It can seem daunting at first, having to think up a pattern to play around the kit, especially when you are still learning how basic rhythms work.
In an attempt to streamline at least the rhythm aspect of writing fills, I created a system called “Number Cells.” It’s as easy as 1,2,3,4... quite literally! The numbers all correspond to certain types of notes, substituting the relevant number for the amount of notes in an individual count, as follows:
Quarter Notes = 1
Eighth Notes = 2
Eighth Triplets = 3
Sixteenth Notes = 4
As a beginner, it’s easier to start off with combinations of quarter-notes (1), eighth-notes (2), and sixteenth-notes (4) only. If you are creating a fill in a 4/4 time signature, then that gives you four combinations (groups, beats, counts, etc) of notes. For example, you could create a fill that uses eighth-notes on count 1, then sixteenth-notes on count 2, a quarter-note on count 3, and eighth-notes again on count four... this would create a number cell of 2-4-1-2. Another example would be to use a quarter-note on count 1, sixteenth-notes on count 2, a quarter-note again on count 3, and finally eighth-notes on count 4... this would create a number cell of 1-4-1-2.
By creating number cells, you have made the task easier for a new student to create a fill, by eliminating half of the guess work. Once they have decided on a number cell to use, and have thus created a rhythm pattern, all they have to do then is figure out what to play with it. Of course, they still have to be creative, and maybe even spontaneous, but their job is now a bit easier. Here are some written examples of basic fills created using a chosen number cell:

Once the student has mastered this process, then it’s time to introduce eighth-note triplets (which is represented by the number 3). Of course, this makes the overall rhythm more complicated, and the drummer-in-training should have a good concept of how triplets flow with the other notes (especially sixteenth-notes). Here are some written examples of more advanced number cells, incorporating eighth-note triplets, as well as some eighth-note rests on the 2’s:

Hopefully, this system of creating fills will make it easier for drummers beginning the fun (but scary) task of writing their own. After this, things get more complicated, but by then the student will have a better understanding of how fills work.
Good luck!