I made this video it may help. It sounds right but the plastic should pinch the o rings to th pin which keeps it fairly tight
IMG_0551_x264.mp4 video by fright88 - Photobucket
As for written instructions. Take apart the UFS. Unscrew the metal nut which removes the pin assembly. Then take apart the pin assembly. You should have 6 parts 3 o-rings pin metal nut and plastic piece.
Now take the pin and put on the larger o ring. Then put the metal nut on the pin (threaded end away from the flat head) Now put the 2 tiny o rings on the pin and slide them down to the metal nut. Finally put the black plastic piece on and screw in the metal nut. It may take a little working to get the o rings inside the plastic but they will go in. Once you finger tighten the metal nut thread the whole assembly back into the connector.
If you followed these directions and the pin still just falls out you may need new o rings.