Can you do a standing bhs?
I usually like my gymnasts to be able to do a standing bha as a pre-requisite to learning ro-bhs. I think it is a 'mental' thing. If they are confident enough to do it standing, they shouldn't have a problem linking it to a roundoff.
However it isn't something you absolutely have to be able to do in order to do a ro-bhs so if you can't, don't worry.
Along with all the other good suggestions above I'd like to add a couple:
Pile a couple of mats up at the end of the floor, until they are about bum height, then roundoff towards them and jump straight back to land on your back on the mats. You will need to make sure you judge the distance correctly. You want to be jumping backwards in a tight arch. When you land, make sure your feet haven't lifted into a pike.
Another drill I've picked up is to place 2 or 3 safety mats in a line along the floor with a springboard at the end of them and then cover all the mats with a 'roll mat' (a long thin mat which is usually rolled up for storage - I guess you could use anything though because I think the point of it is to cover the joins in the safety mats). Roundoff, placing your hands on the springboard, then land the roundoff on the mats and bhs.
It sounds like a funny set up, but it really does work. Many of our gymnasts learned theirs this way last summer! However, I would just warn you that it might not be a good idea to do a lot of these as the soft mats may hurt your wrists after a few.
It is a good confidence builder simply because the mats are softer than the floor!