I recently did the leatherette for a Hassy A12 back.
1) I peeled off the original leatherette, where the glue is loose and slimy.
2) I use 97% alcohol to clean up both the back of leatherette and the metal parts, to get rid of all of the goo
3) I apply Pliobond P141 only to the metal side, wait for 5-15 seconds to the clue to thicken, and apply the clean leatherette
4) The glue will set in a couple of hours
5) The glue smells bad, but works well
Glue used:
Ruscoe P141 1oz Pliobond Adhesive, Amber