Start with whatever bike you have. If you don't have one now, you might look at using an old Triumph. I won't say it's easy, but it is different.
Find a few old 1970's and 80's chopper mags like Easy Rider, Iron Horse, Street Chopper and such. You will find out what choppers are. Don't waste too much time watching Orange County Chopper reruns or Chopper Build Off.
Set back and drink a few 6 packs or pots of coffee, read the mags and make a few sketches on the shop floor, the walls or paper napkins or even the old lady's kitchen table.
Buy a welder, drill, bits, grinder, files, box full of tools and more beer or coffee. Learn how to use the welder and other tools while at the same time learning how to build your ride. Drink more beer or coffee. That would be a good start.
Anybody can buy a bunch of chrome parts and bolt em together, but not everybody can build a chopper and say "I built it".
It's like carving a bust of Abe Lincoln. Buy a block of marble, some chisels and hammers. Look at some pennies and start removing everything that don't look like Abe Lincoln.