If your tree is currently a single whip, I would focus more on limiting its central growth (nip the tip to encourage branching). Once you have the branches coming out where you want them, then you could just graft those particular branches over to your chosen varieties. This might be the easiest method.

If you want to do the bud grafting technique anyway, and your central leader is not too large, then you can easily do chip bud grafting (or T-bud graft when the bark is slipping) anytime the tree has leaves. Do it right now with dormant scion wood, and you have a very good chance at having those buds grow into branches this year. Do it in late summer using summer-dormant buds, and they will overwinter and sprout next spring. This method is a little trickier at times.

There are even other grafts that let you install a totally new branch on a main trunk where there was none. But it sounds like your tree is not large enough caliper to do that right now. Unless you have some very small caliper scions available to use.

Many ways to skin a cat, as the saying goes!

Hope that helps some.