Aye breh if you really wanna do this, then go about it this way:
1st- Buy the morning after pill, it's gonna cost $40 but believe that those $40 are way cheaper than alimony and college tuition in the long run. Along with attempting to be happy with a shotgun marriage. Just buy the pill breh, don't even think your way around this one. If you don't get the pill then know that step number 5 (further down) will be even worse.
2nd- Don't let yourself go, always remember to slow the fukk down when it gets too good.
Stroke management will allow you to last longer, so just remember: long and slow strokes when it gets too good. Fast and short strokes when you're cool. Fast and even-shorter-strokes when you wanna keep the rythm (so as to focus on the pelvis thrust which takes your mind from the orgasm).

3rd- Understand that the voice in your head the other breh was talking about, the one telling you "

Just nut inside breh, this is too good to get out now

" will be pretty loud when you're mid-stroke. Which is why
step 1 is important.
4th- If you do come, just remember to enjoy the moment. Don't freak out, and if she freaks out remember to help her calm down (let her know you bought the pill before sex, just for this step to be smoother). It ain't the end of the world if you've got the pill...if you ain't got the pill then...you're on your own breh.
5th- Ask her to let you know when her period comes (schedule-wise after you and her go raw) because
if something goes wrong, the sooner you know the better. This shyt is crucial homeboy!!! And know that until her period comes, you're gonna be shook as fukk with uncertainty, just 'cause you ain't gonna know if the pill worked or not. That's just the way it is though, the post-nut-clarity is a bytch.

Good luck.