I use a number 3 Phillips head screw driver to push the seal in. The shaft of the screw driver is just a bit smaller than the push rod so it won't hurt the seal. Get a fender washer that just slides down on the screw driver. If you don't know a fender washer has a small inner hole with a very large outside diameter. Get one at the hardware store, ask one of the older fellows, he'll know what it is.
Slide the washer on the screwdriver, slide the seal on the screwdriver, now insert the screwdriver into the hole the pushrod goes into.
The shaft of the screwdriver helps keep the seal going in straight. The washer if bigger than the seal hits the case and keeps from pushing the seal in to far. Just a gentle push will be enough. No hammer.
A bit of Yamabond on the outside of the seal lubes the seal so it slides in easier. After the seal is inplace the Yamabond helps seal the seal to the case.
Oh, one more thing. On the edge of the seal hole the edge is very square and sharp. This is what F's up the seal when you push it in. You need to break this sharp edge. A pocket knife, a deburring tool what ever you have. Just bevel the edge a bit, so it won't cut the seal.
Other than that the instructions on Mike's XS site are very good.
Leo