The easiest way I've found to install a caliper piston boot is as follows:
1. Put the boot onto the *back*
of the piston so that the edge to be
inserted into the caliper groove hangs off the piston. The boot should be
pretty well stretched out at this point.
2. Gradually maneuver the piston to insert the boot into the groove in
the caliper. Use a small screwdriver or similar tool to *gently*
final half or so of the boot into position in the caliper groove.
3. Once the boot is in place in the caliper, gradually slide the piston
back into the boot until the boot engages the groove in the piston.
I know this sounds pretty simple, but it's still tedious work. I usually
need to try two or three times to get everything just right. If you're in
doubt, back up a little and make sure everything is just so. Don't force
anything. Everything should go smoothly when aligned and fitted properly.