Wait til either just prior to buds actually opening (rather than just being seen on the tree), or else after they've opened and leaves have matured somewhat (by May/June depending on what you see). Air layering is pretty basic, with only variations on the 'theme', but I'm sure if you research a bit you can find many ideas.

Larke's answer is good, assuming you don't have a evergreen boxwood variety that keeps its leaves all year round.

Here's a good article by Harry Harrington that talks about the basics for the different types of layering and gives some actual examples, should explain everything you need to know. Feel free to check out some of the other articles on his site. I found some topics interesting an learned a thing or two.

Good Luck
-Mike