You must apply online using usajobs.gov. If you have a 4-year degree, you should automatically qualify. If you're a military veteran or have prior service in any federal agency, you will be on a preferred list and be contacted before those who do not. You'll receive an email telling you to report to the local campus, where you will get fingerprinted and fill out a bazillion forms. Make sure to know where you've worked and lived for the past 10 years for the background check. Make sure you are current and paid in full for both state & federal tax obligations.

I went to the IRS in-person hiring event. It was a really simple process of registering and fingerprinting. No actual interview questions were asked. The whole process took about three hours.