Carefully clean out the seat and feel it with your finger as well as the beveled edge. If you feel any burrs get some 600-800 grit wet sand paper and smoothen it out. Clean it again, then rub some spit on the outside of your seal and position it over the hole. Get a socket or something of similar diameter as the outer diameter of the seal and start tapping it in with a rubber mallet. If you feel it going sideways stop, remove it and try again.