The first thing is it can not be said theoritically definetly practically so how to bowl a flipper?
It is also called as skidder . because it skids. generally the thumb ,fore,middle,and remaining 2 fingers will be on seam. The basic thing is that u need to remove fore and middle fingers at the time of dropping the ball ,so that it skids and pitches litle bit shorter and goes straighter at a speed of 100 to 110 kmph .

Normal leg break is bowled by rolling a wrist,fingers .I mean the ball is released at the level of shoulders then it breaks from middle and leg to out side the offstump.similarly flipper is bowled by removing the fore and middle fingers so that it has to skid and remaining fingers has to be in same position continuing to that th ball has to be released at a position between shoulders and spinals cockyx region.Then u can definetly can get flipper.
The flipper word has come from dolphin flaws ,i think u have watched some where in the sea dolphins move so fastly with the help of there flaws similarly the same principle is been used so that the ball moves so fastly so that the batsmen cannot know and find , it comes so fastly and straighter and definetly he will be demolished if bowled correctly