You want to graft dormant scions onto actively-growing understocks. Best if scions have at least 2-3 buds.
I do mostly a simple bark graft, but did simple splice/whip grafts when I first started out, with near 100% takes.
Aftercare is the most important aspect - you've got to check almost daily - and keep buds/shoots below the graft rubbed off, or the rootstock will direct resources to 'its own' material, and the grafted scion will decline/die.

I have had success whip and tongue grafting American persimmons. Wait until the first leaf is showing on the understock. Out here that's May-way later than apples, but as you see, it's the right time for the persimmon as it starts to grow. If you graft at the same time as apples it will be very unlikely to work. I have a different climate than Lucky. Rubbing out shoots is important, but not as crucial here. One thing that killed one of mine out here in our dry summers was just 3 days of hot weather when I didn't water it for the weekend.
Good luck.
John S
PDX OR