Arrive and sign in. Bring a current/valid driver's license and social security card/a copy (scan and print). Wait in the waiting room for it to start, and then they go down the list. It takes a bit, so you may want to go over answers to possible questions or bring something to read/occupy your time (and your phone, on silent). When you are called back, you talk with them about the job and give them the copy of the driver's license and SSN, while also signing/initialing items. The questions I was asked were focused on things like past work experience, could I do this as an RCA (lift a certain amount, etc), what is your vehicle/can it be used to deliver mail (meaning driver's seat is right side, instead of left) or will you have to straddle the middle and drive carefully, then switch back into driver's seat to drive to next location, and repeat. The interviewer answers questions you may have, and informs you of things such as when you can expect to attend orientation, if you are selected.
I had attended several interviews before finally being offered a position and accepting it. You then are scheduled to attend Orientation, which lasts a week. This includes going over the material of what you will be doing on the job (there's a handbook), what you are supposed to wear, questions, videos, etc. The orientation I attended was mostly RCAs, with 2 other people in other positions. We were separated after 2 days, or so. One of them went to work right away to learn more, the other never showed up for the rest of his orientation and we couldn't contact him. We then watched more video lessons with questions to answer, practiced collecting and casing mail (which was the first time I had done this), practiced driving the LLV and using it to deliver mail (as well as the safety check and how to park it). Then came the "Shadow Day", where you are scheduled to follow someone working at the location you applied for/got an offer for. This more than likely ends up being the route you work on. They add a seat in the back of the LLV, and this was in the summer, and it was HOT. There is no air condition in these vehicles, and no windows so I barely got any relief. After it was over, I left. You are also meant to keep track of your time and "clock in/out", as well as record a break, if you take one. The latest they want you back is at around 5/5:30 pm.