Step 1: Dump That Old Bumper!
So...it's not fun, but that old bumper has to come off, so you can start making some design decisions, and taking some measurements. Depending on your specific vehicle, it's age...and how much road salt it's seen....you may require more of your favorite rust buster lube than you think (WD-40, PB Blaster are my two fav's).
First, locate all bolts affixing the bumper to the frame. Soak those suckers in PB/WD-40. Wait 10 minutes...and soak them again.
Unbolt everything, and drop the old bumper away. Take your time....those bolts are probably large, and you probably can't break them off, unless you're sporting 26" arms, and a 'born to kill' tatoo.
As for your old bumper, I'd suggest storing it, in case this project goes wrong, OR just in case you decide to sell the truck later, and fear your new custom dragon tooth bumper isn't going to be a 'good selling point. If you have plastic trim/moulding/etc surrounding your bumper, be careful removing those...you may need them later, and just jerking them off is sure to crack them...take your time.