I was assuming you were trying for a 1RM, but my advice is pretty general for any sort of PR attempt. I think
If you don't know what your 1RM is or don't even know where it would be, just ease into it. Hell, even if you do know what it would roughly be you should still ease into it. Coming off your warmup start doing sets of single reps that you know you can handle and keep adding small increments to the bar. You should leave plenty of rest between sets, time is your friend in this sort of thing so don't be afraid of a 5 min break between the heavy sets. Just keep adding small increments and you'll find your max. It doesn't do any good to bench (random number) 235x1 and then miss 250x1 on your next attempt, you've got a range of 15lbs there. It might not seem like much but even a couple of pounds can mean something when you're playing with single reps.