get on a trampoline with a spot and start working on your backhand spring .... after you can do that by your self get on a long trampoline and get a spoter do your round off and have the spoter help you do your back hand spring

The last half of the round off is basically the same as the back handspring. And you already can do the back bend. The other two part is 1. Hand stand and getting 2. to the hand stand by jumping and throwing arms to the handstand making it a back handspring.
1. For the handstand your will have to be able to hold your body weight for at least 2 seconds. You only need 1 sec., but the second is for good measure.
2. Break down the round off. From the cartwheel or hanstand positon you will snap your feet down and shrug through your shoulder to get your hands up to go back into the back bend to the handstand position. Now once your hands comes up to your ears your need to watch (w/ your eyes) your hand go over into the into the bridge (back bend) position then to the handstand.

Now a drill you can use is jump and throw your arms in the arm (keeping arms straight). When you are doing this drill correctly arms will be by your ears and will be about a foot off the ground. This drill will provide you w/ the timing for the jump and arm placement for the back handspring. The rest is up to you. It would be easier to use an octagon or barrel to help get over the not seeing while going backwards. If it seems difficult to do or accomplish got to a gym to further assist in drills to get to the handstand backwards. This is the most important part of your tumbling career.

Do a roundolph Pause Back handspring