Scion collection can be a bit tricky, but not all that complicated. I've found that somewhat vigorous shoots near the top of the plant on wood from the previous spring gives me the best success. Late summer shoots can work, but they are often damaged by cold and don't have as much energy stored as a spring or early summer shoot. I like 2 or 3 bud pairs, unless it is a dwarf in which case I sometimes go up to 4 pairs. I've found that twiggy thin scion doesn't take as well as fatter stems (although stems fatter than the rootstock are not generally desirable either). I'm still working on the post graft care myself as I'm not that experienced in this either. But I can tell you that you need to wean the grafts off of the humid conditions into normal conditions, you don't want to just take the bag off and throw them out in the sun after they leaf out. I don't bag my trees, I just keep my greenhouse at high humidity and keep the tops misted occasionally (and I apply a fungicide for good measure). I definitely have a higher rate of success with bagging, but it is a bit tedious and more time consuming with larger numbers. It also makes the weening process a bit easier because the conditions don't change much until later in spring when I take the plastic off of the sides of the house. Hopefully this helps a little. I usually wait at least a couple of weeks after the graft takes to remove the top of the understock. Some dwarf and weak varieties (like some variegates) I wait a bit longer, or I remove the top in two steps. Some people bend the top of the rootstock over and break it just above the graft to encourage a transition of dominance to the scion without complete removal of the stored sugars in the top of the rootstock. This might help push a little more growth and faster healing but I haven't experimented with this enough to know just how effective it is.