Basically, a staffer on the committee found me online, was interested in my ideas, and asked for a phone call. A couple of days later, he scheduled a second phone call, this time including the committee chairman's staff from the opposing party.
I got a formal invitation to appear with about eight days' notice, along with a request for written testimony on specific topics. You pay your own way to get to D.C. for the hearing.
The testimony was due 24 hours before the hearing, so that staff had a chance to review it and plan their questions.
In the hearing, there is a two-hour block of time, and senators each get five minutes to ask questions, alternating parties, in order of seniority. Some of them want to know stuff, others want to read a statement into the record. You get a glass of ice water and a little countdown clock at your desk that tells you when the five minute blocks of time are up.
At the start of the process, the three witnesses each get a five minute opening statement. In my hearing, we had a former diplomat and expert on the GDPR, and someone from the financial industry. The hearings are open to the public and if you are in D.C., you should attend one! It is quite a spectacle!
I'm very thankful to Matt Blaze and others who wrote about what hearings are like, to help me prepare for this. And I am happy to answer questions in turn.