This is most easily done in a singles match, so as to make sure nobody attacks you while you're taunting or your opponent as they're waking up, but can be done in any mode.
Basically all you need to do is make sure you have a finisher ready, take down your opponent and hit up on the D-Pad to activate your finisher. When the opponent gets up, hit your finisher and pin them. Assuming you've worn them down, it should work MOST of the time... if not, build up another finisher and do it again.
Hints:
- You can set up your character to have infinite finishers in the selection screen, after you click on them the first time. Doing so will not only save you time rebuilding if you fail a pin, but will let you hit finisher after finisher which will wear your opponent down faster.
- As long as you're using infinite finishers anyway, you might as well go for the OMG moment achievements too - the double shellshock, putting someone through the announce table, and breaking the barricade in 2 places in one match, as these are all easiest to do with infinite finishers anyway.
Note: It does not matter who you do this with, but some people have reported it not working on their first try. If you don't get it on your first attempt, start a new match and try it again. Even those for whom it doesn't work right away seem to get it on the second try.