I've used a drift punch, but I think I've used mostly an old screwdriver of mine with about a 3/16" dia. shaft that I ground off flush/square. (The tip was hatched anyway) Just drive home with a small hammer.

The trickiest part is always getting the 'right' spline slid on so the holes are aligned correctly. The same punch/rod can be slid through the assembly with no interference if you've correctly lined them up. I usually try to have the hole lined up vertically so I can 'eyeball' it first.

Good Luck. Have fun.

-Mark