Actually you need to work on your flexibility - and strength and control - first, then learn how to do them. Quad and ab strength is very important. Be aware that some people will never have the flexibility required to do this safely - not all range of motion issues can be improved - like bone orientation - and even those who do can still injure themselves and significantly shorten their dance lives. Also be aware that a "good" back bend takes a little from each joint. A sharp back bend (like folding a bit of paper) is not only ugly but causes huge stress on the lower back.
Still want to go ahead? Then find an experienced teacher to show you how (I had the good luck to have Hadia show me how back when I was young, strong and flexible - not something I'd do today!). You may want to look at a jazz or contemporary teacher as many belly dance teachers care for their students and their own bodies too much to teach this
This is not something you can learn remotely.