Hey -

Well, first off, I can't do a roundoff backhandspring back tuck. BUT, I bet you can find some instructions in the Weplay cheerleading tumbling library. Check it out:

http://www.weplay.com/youth-cheerleading/drills/skills/tumbling

begin practicing the bridge and or bridge kickback then practice catching ur self on a fronthandspring this should incerase ur chances of landing it safley with strong arms .....

Your question is a tough one without knowing how well you are able to do a round off back handspring. I would need to know if these two skills are technically sound before moving forward to doing a back tuck at the end of them.

I would strongly recomend you find a gymnastics club in your area that has open gym time and a coach available to help you. Additionally, it would be best if they have an in-ground pit filled with foam for you to tumble in. Spending a little time with someone like this would be very beneficial. They will keep you from developing bad habits, if they are good, and can decrease the amount of time it will take you to learn these skills in combination.

have a good spoter spot you

Keep trying make sure you work on your rebound so you have enough power to through that stuff. it's complicated but gets easy with practice

Assuming you have a strong roundoff backhandspring rebound....I teach standing back tucks first. Assuming you have that also, your rebound is your set, always allow yourself to rise first, then pull your hips forward upward over your head with your knees bent. Always flip leading from your hips, not pushing back with your head or arms. Practice on a trampoline first, then take it to the floor when you have it mastered there.