what is the best way to learn flip turn

You don't say whether or not you have received any instruction. Presuming that you have, the most important thing to do is to keep your speed up as you approach the wall. The most prevalent problem with people learning the flip turn is their lack of confidence as they approach the turn, which results in a drop in speed which results in failed turns, since you lose much of your momentum in the act of slowing down. Swim at the wall as if you are going to swim right through it. You will not have any means of propulsion as you execute the turn and it is truly amazing how immediately you can slow down just by ceasing to swim, so make the most of what you bring to it (the turn). You will also have to judge where to flip over in order for your feet to meet the wall at the completion of the flip. Don't worry if you are too far away the first time. Practice will allow you to make the judgment automatically.

You don't say whether or not you have received any instruction. Presuming that you have, the most important thing to do is to keep your speed up as you approach the wall. The most prevalent problem with people learning the flip turn is their lack of confidence as they approach the turn, which results in a drop in speed which results in failed turns, since you lose much of your momentum in the act of slowing down. Swim at the wall as if you are going to swim right through it. You will not have any means of propulsion as you execute the turn and it is truly amazing how immediately you can slow down just by ceasing to swim, so make the most of what you bring to it (the turn). You will also have to judge where to flip over in order for your feet to meet the wall at the completion of the flip. Don't worry if you are too far away the first time. Practice will allow you to make the judgment automatically.