Spit on the tip of your finger, lube the inside of the hole, and then wiggle it around and twist a bit back and forth as you apply pressure.
No, seriously!
After lubing the bushing. I like to hold the shock clevis with one hand and with the other I apply pressure to the upper mount to force the rubber grommet over the upper mount stud. With the hand that is holding the lower end of the shock, pivot the shock forward and back, which will twist the rubber bushing around the mounting stud as you apply pressure with your upper hand. The lower hand can also push the shock towards and away from the centerline of the bike, sort of "rocking" the bushing onto the stud.
Make sure the mounting stud doesn't have any sharp edges or burrs or it'll tear up the rubber. You do want a tight fit up here there, but you shouldn't have to Hulk it into place or use any tools.
mystic_1