Jim:

Take the scions in late January or up to the end of February when they are dead dormant. After that, keep them that way in refrigeration in a plastic bag with only a slightly moist paper towel. You don't want them to dry out, but you don't want them to mildew either.
Graft when the crab is waking up and starting to push leaves in April. Your scion will still be fully dormant, and that's the way you want it.

Don Yellman, Great Falls, VA

You might find scions that big -1/2"- to be a bit much to work with. I think "pencil sized" is plenty big enough, and a lot of time you'll find you can work with 3/16" or even smaller if your tools are sharp. You can readily graft a smaller scion to the larger rootstock, and some grafters will put two scions on a single cut, eventually removing the poorer. No problem putting a 1/4" scion on a 1/2" branch, for example. Line up the bark side for cambium alignment.

Water sprouts are good scion sources.

:-)M

I was wondering cause I have a yellow apple that the
wind broke up pretty bad a couple of years ago. I've
been pruning some each year and have lots of water
sprouts and a crab apple that the deer ate the top off
of.The crab is only about a 1/2" in diamater. Thanks for the help.

Jim Zone 5 MI