Start off like you would for a forward traveling gymnast cartwheel. You can take a small jog or run into it for more momentum.
Roundoff
Optional
none
Performing tricker: bam.defocus
It's a gymnast cartwheel where you push off with your hands and snap both feet down to block into the next trick.
You need a gymnast cartwheel
The direct prerequisite to the roundoff is the gymnast cartwheel. The roundoff finishes with you facing the way you came from, just like a gymnast cartwheel. Work on it and try to land with both feet simultaneously. Then add a push and it's pretty much a roundoff.
As you reach down swing your legs up one at a time. Try to bring them together when your hip is directly above your head or even a little bit earlier.
Snap both legs down, while you simultanously push off with both hands.
Land upright and try to hit the ground in a 45 degree angle...
...to get the most optimal block for the next trick.
Tips
Enter your roundoff with slightly bent arms, so you can push off through your elbows.
Most of the time you'll use the roundoff to punch into backflipping tricks but you can also use other transitions to get creative. Example: Roundoff rev/pop corkscrew.