easiest thing to do is go to ATF.gov website and print off the form (a couple copies) and read their FAQ.
basically, you just need to fill it out, with pics and get your finger print cards, etc, then get your chief LEO to sign it. send it in with a check for $200 to the address listed on the form and wait 6 months for the bureaucrats. once you get the paper back with a stamp on it, get a hacksaw and shorten your rifle.
you will need to figure out what gun you want to shorten first, and how long you want the barrel to be, and write the serial number and length on the paperwork, of course.