Slit a piece of ABS plastic pipe and hammer it home. Make sure to use a sledge and don't be afraid to beat into submission until the snap ring easily falls into place. If you have to push in place, the seal is down far enough.
Getting the center bolt out w/o the damper tube spinning was the hardest part for me. And then keeping everything clean was the 2nd hardest part.
Make sure the inner tube is perfectly smooth and if there are any burrs, sand them with super high grit sand paper. I replaced both mine and about 1000 miles later, I got a rock chip burred my tube and ripped up my seal.