First, you need to know where to flip. Start swimming backstroke from one end of the pool and count your strokes from the flags. If your pool does not have flags, have someone stand on the side of the pool five yards from the wall. This stroke count is usually anywhere between 2-6 strokes depending on your length and speed. Repeat that test a few times just for accuracy. Then, subtract one stroke. This is when you will turn over and prepare to flip. For instance if my stroke count was five, I would turn over on my fourth stroke.
If you cannot flip over for some reason, try the corkscrew drill. While swimming backstroke, reach one arm across your body into a freestyle stroke. Then with the opposite arm, flip yourself back onto your back. That is the same motion used in the backstroke flip turn. Once you have flipped onto you front side, you cannot take another stroke no matter how far you are from the wall. Your arms must stay at your sides, but you can kick. Approach the wall and tuck your chin into your chest and breath out through your nose. Do a somersault and finish on your back.
At this point, your feet should be flat against the wall and ready for push off. Bring your arms above your head into streamlined position. Streamlined position is where you are stretched out as long as you can, and your hands are stacked one on top of the other above your head, and your arms firmly pushing against your ears or the side of your head. Stay underwater until at least the flags. While underwater in streamlined position, you can do butterfly kicks with your knees and feet together, gathering momentum from your entire body. Once you have surfaced, you can begin stroking.
Common mistakes in backstroke turns are flipping too early or not bringing your arms up fast enough. All it takes is practice. It took me probably 1000 times to finally master it. Don't give up if you really want to do it. Just like everything, practice makes perfect.