Seat Covering Help...

OK - HELP - I have PUI seat foam covers and there was NO instructions with them - matching them up with the old ones it seems like I should shave off the top - however i dont want to do this without real true instructions.... secondly even using the plastic bag method - I just ripped the bottom portion of the vinyl... my wife can patch it and its where you wont see it, but still I'm getting close to tossing $600 down the toilet... HELP....