The name of the trick makes it pretty clear: during an fs tailslide, you perform an fs rotation in order to slide on the ledge with the tail of the board. Requirements for the fs tailslide are a proper ollie and, even better, a proper fs 180. How to pull off a perfect fs tailslide is shown and described here.
FS TAILSLIDE - STANCE: REGULAR
Approach the obstacle at an almost parallel angle. Staying a tiny bit diagonal is recommended. Your feet should be in ollie position and you should be crouched down.
Right in front of the ledge, pop off as high as possible. The tail must clear the edge of the ledge. Turn your lower body by 90°. Your shoulders, however, should stay parallel to the ledge and not rotate with the body. When you get to the highest point, it’s time to put pressure on the back foot and take the weight away from your front foot so that you can get into the proper tailslide position.
If you apply medium pressure on the tail, you'll slide all the way to the end of the ledge. It's recommended to lean back a little.
Make a quick ollie motion in order to exit the slide. Turn your body back into the starting position so that you land parallel to the ledge.