I don't remember where I learned this from and my process probably differs a little or a lot from the original but my students love this game and it's been so fun for a "brain break" when we are in the middle of some serious work on instruments to just say, "Crazy 8th's" and watch 'em run to get drums and tambourines (my fourth graders are such great kiddos!!). I'll start by keeping a rhythm on a djembe or the bongos and then, once i've established the steady beat, they'll start to count to 8 and march around the room. Each pattern of 8 you put an accent on one number; the first time it's going to be ONE, 2, 3, 4, 5, 6, 7, 8, then 1, TWO, 3, 4, 5,6, 7, 8, then 1,2, THREE,, 4, 5, 6, 7, 8, etc. They only play on the accented beats; march each beat.
The first time will go slow- they'll need to simply march and count to 8 several times, marching on each beat; then stop the class and say, OK, let's emphasize the number 1 and still march on every beat, then stop the class, demonstrate the accent on beat 2, then beat 3, etc. It helps to project or write the numbers 1 through 8 then you can point to the next accented beat as the students progress through.
Once the students can do this up to 8 smoothly,(which takes a little while), go backwards.. yikes! Fun!!!
Once they get comfy with this, continue to only play the accented beats but change the marching (locomotor) to standing (non-locomotor). in other words, step and play ONLY on the accented beats. Much trickier! Have fun with this!