The obstacle can be pretty much anything of any size. But something like the corner of a curb is best for beginning. If it comes to any kind of point, it will help.

Ride towards the obstacle at the fastest speed your comfortable with, the faster the better. If what you are planning on bonking is straight, go towards it at the softest angle you can. But if the obstacle comes to a corner, just ride at like you would to bunnyhop over it.

Bunnyhop, like you're going to hop onto or over it. Remember to hop sooner than you would to get onto it. Right before your front wheel passes over the obstacle, push down on your handlebars, so your tire bonks the obstacle. You don't have to worry too much about lifting your tire off of the obstacle; just turn your bars slightly away from it, and you will just roll off.

Whipping the back end out is what looks like the hardest part of the trick. But its not hard at all. You don't really need to make much of a effort to do this. The only thing you may need to do, to help whip the back end out, is just slightly shift your weight back, away from the obstacle. Do this right as you push your front tire down onto the object.

To ride away from the nose bonk, all you need to do is lean back and away from the object, and turn your bars away from the object.

This isn't that hard of a trick to learn, as long as you try hard, it shouldn't take any longer than an afternoon.