I just recently learned how to a burnout and this is what I do...
I rev up to about 3000-3500k and dump the clutch and then put pressure on the brake with that same foot (left foot). When you dump the clutch it will make the tires spin and by pressing the brake it will keep you from going forward. Just keep giving it gas the whole time but hold the rpms, I usually hold them around 4500 or a bit more. If the backend tries to slide out just press on the brake harder. It only took me a couple tries to master it but I killed it probably 6 times in the process lol. Just gotta get use to doing it cuz it feels very weird at first.
Here's my clip of it:
burnout at the track (not the best but im still a noobie

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As far as powershifting goes, tried that for the first time the other night at the track.. if I were you I wouldn't try it just yet. I would first get use to doing the burnout, stanging, shifting, and just driving down the track. Once you think you're ready you could try to powershift. The main thing is to NOT miss a gear!

It could be very dangerous for your car because your RPMs climb when you shift instead of drop, you miss a gear and they still climb.. well you can imagine would the result would be
Good luck to ya!! Post your times!