I regularly get together with some friends that all use a straight razor. It always a cozy evening, wives included. We chat a bot about various topics, while we hone razors, admire each other's latest razors, soaps, brushes, strops, etc.
We usually end up playing barbershop, shaving another with the full treat, hot and cold towels included. Occasionally some female legs get shaved as well. It's always fun. We drink whiskey before and after the shave, but every one watches out with drinking prior to him handling a razor.
If you want to shave other people, I'm sure Gary will chime in to correct me where needed and to add more tips, but I'll sum up some things that I think are important.
1. you need something that has the functionality of a barber chair. With our group, we use a real one, but one of those plastic patio chairs with armrests and folding back would work well too. Arrange the cushion so that the person being shaves can sit/lie in relaxed position with his head resting backwards a bit (so the neck skin stretches nicely)
2. Make sure the razor is absolutely shaveready and sanitized.
3. Trim his full beard with scissors to a length where you can get lather at the base of the hairs. 2 cm or so is no problem.
3. Get a towel in his collar. Lather him up and apply a hot towel. Press the towel kindly in contact with the cheeks and neck area. While to towel does his work, strop the razor.
4. Remove towel, re-lather. (Make sure the lather is hot by using a good shuttle or floating a bowl in hot water) Spend some time lathering.
5. Time to shave. Please refer to a good barbering textbook for methods how to shave another person.
6. Talk to the person you shave, ask him to help with making facial expressions that stretch certain parts of the face. Ask him about the comfort of the shave, etc...
7. After the shave, rinse his face with a bit of water, rub with alum block and apply a cold wet towel for a few seconds.
8. Put on some aftershave, and don't forget the "towel fan"
That's more or less how we do it. So far, an occasional small nick aside, all things went really well. The first few time we couldn't manage to give shaves as close as what we get when shaving ourselves. It's better to stop with a bit of stubble than to cause skin irritation. If your brother hasn't shaved for a long time, expect his skin to be on the sensitive side, so I would keep it at 2 passes. Keep the razor angle on the low side and be very cautious to pressure.