How do you keep the boot clean of grease as you put it back together?
The secret is, don't put greasy tools or the CV's in the boot!
Or put lots of news paper down on the carpet first, lol
Jokes aside, not completely sure what you meant? There is no problem getting grease on them or in them.
Just clean any on the outside when installed.
The critical points are to not overfill the joint with grease (usually the packet they give you
will suffice) and also not to over inflate the boot with air when tightening the s/s bands.
For example, you wouldn't want the joint slid fully to the outer end of the shaft, and then
tighten up the boot, as when it slides back along the shaft it would blow it up like a balloon
putting lots of stress on it and causing it to fail prematurely.
Conversely, you don't want to slide the joint all the way on, then tighten boot bands
as this will create too much vacuum at full stretch.
John