Greg Daley says the only time to graft cherimoya is in spring and when removing scion wood, store it in the fridge until leaves absciss revealing new buds and then graft. Cut the leaves off and leave a stub before putting in the vegetable crisper wrapped in a plastic shopping bag. I pushed the stubs off with my finger to expose underneath prior to wrapping the scion and graft in parafilm. I use the green 1cm wide tape which the florists use. I grafted using a very simple cleft graft. It was as easy / easier than grafting an apple or nectarine tree. Ref: Daleys website