Once I get the edge of the seal hole deburred. I take a Phillip's head screwdriver #3 is just a bit smaller than the rod. Get a fender washer that just slips on the screwdriver and has a larger Outside diameter than the seal. A bit of grease on the screwdriver to help things slide. Slide on the washer, then the seal.
I coat the outside edge of the seal with Three Bond, the same stuff I seal the case halve with.
Slide the tip of the screwdriver into the hole where the rod goes in the shaft. This helps guide the seal straight in. The waher acts as a stop so you can't push the seal too far.
Clean off any excess sealant from around the seal.
Leo