This is always the hardest step. Deciding where the head should go. You want it to look good, but you must remember to keep as much gap as possible in order to get hook-ups. This is easiest by simply moving the head around until you like the looks.
Then roll the film back far enough to put a dab of glue on the fly head.
Then hold down the fish skin while it dries. Using two fingers helps the head to be concave over the fly head. This will make the two halves meet up better in the middle.
The worst thing about these heads is that they take time. Go slow, use a little super glue and do the head in pieces. Once you have done a few, the process will be easier.
PLEASE try not to glue your fingers together. Use a clothes hanger or something else to hold the two halves together.