If you have a certain rock you want to place it on, you can bore out a small hole, deep enough and wide enough for it's base to sit down in (not too tight and not too much empty space either around the base). Place the base down in the hole and leave it to attach over time. The problem with this is that it still might come out of the hole depending on your flow and other critters that move around at night. I sometimes would take a piece of pvc and place it down over the head of the coral and sit the pvc around the base as to try to keep stuff from moving the base out of the hole and to help block flow that might blow it out. Sometimes its best to do this in you sump or fuge or other low flow area till it attaches. You can then just move the rock and leather to the main tank. It should attach in a week or two. If you do the pvc piece, you just want to be sure the opening will fit over the head of the leather, which usually isn't too hard as they will shrink up if you tick them off or pull them out of the water. Note: I always used to like to use a decent size rock for a base, as the leathers can get very large. I liked having a rock that was large enough that would support the weight of the leather and allow me to more easily move the leather to where I wanted by just moving the large (but mobile piece) of rock it was attached to.
I've done the rubber band thing also. I would poke a toothpick through the base and tie rubber bands around each side of the toothpick around the rock. The problem with this method is that the toothpick can sometimes cut through the leather, so you can't have the rubber bands applying much force on the toothpick. After the base attaches, you cut the rubber bands and can pull the toothpick though and out.
