That generally depends on the weapon and the pylon. I have a drop tank for my F-4 where the tank and the pylon are glued together and integrated into one. For the F-4 missles, I used thin carbon rods to tie the missles into the missle rails on the pylon to keep them in the right position and small screws to hold them onto the pylon itself.

On my Hawker Hunter the tanks have "tabs" that fit into slots in the pylons and a small bolt goes through the pylon and the tab to hold the tanks in place.

Bob