Learn how to do a roundoff whip full combo with tis how-to video.
In order to accomplish this combination you must first be comfortable with doing a roundoff, roundoff back tuck, and back layout.
Start off with a full roundoff landing with feet facing forward.
Once you land, you must immediately begin preparations for the whip.
Then, as you prepare for the whip, make sure you are perpendicular to the ground and that your knees are bent.
As you go into the whip, which is an unassisted back-flip, make sure you push off the ground with enough force to rebound into the full 360 back-flip once you hit the ground again.
If you completed these steps properly you should be able to execute a proper roundoff whip into a full twist.