Providing there is no extra paint to contend with I usually stick them in place with grease then wiggle the arm in. Another pair of hands can be useful aswell.
A tapered drift followed by the arm bolt would also work as the shims would initially not have to be in exactly the right place. I have a small nylon one in the end of the front spindle on the 996 which makes it so much easier to get it back through the forks, wheel and spacer.