Two ways.
1. The hard way: Download and fill out Form 4 from ATF's website. Need to know in advance if the Chief LEO of your town, or county, will sign it off for you. Then, you will need to go through fingerprinting and FBI criminal background check. Once you're all clear, send everything in, along with your $200 for the tax stamp. Remember, you need to fill out TWO Form 4s, because the ATF with send one back to you, with the tax stamp on it. Problem is many, many of the CLEOs will not sign off your paperwork. But, you're not SOL. You go the "easy" way.
2. The "easy" way: Form a Revocable Living Trust. You can either use an attorney who specializes in NFA Trust stuff, or you can use those DIY online softwares, like Quicken Willmaker. Fill out Form 4 (again, TWO of them), but do not have to pay attention to the CLEO sign off / FBI check. That's what the Trust route is all about.
Send everything in, along with your money, to the ATF, and wait.
***************************************************
If you will be "making" the SBR yourself, then use Form 1, instead. Good luck.