Many years ago, I took my first first-aid class, and the instructor was a backpacking group leader and also EMT. At the end of the class, the class members asked him about group first-aid kits. He suggested that the commercial kits were too expensive and that we would be better off to assemble our own. We each contributed a small sum, about $10 (back then), and he bought stuff in quantity at a medical supply house. First of all, you want to consider what degree of injury you want to prepare for, for how many group members, and for how many days you might be out. For example, rock climbers get completely different kinds of injuries. Skiers are different.
1. Triangular bandage cut out of a freshly-laundered white bed sheet. Add some large safety pins.
2. Plenty of sterile dressings. Most are sealed gauze.
3. Some ordinary stuff like band-aids and good tape.
4. Pharmaceuticals, medicines, antibiotic ointments, etc. This is one subject where the commercial kits do better. They have several good items packed into tiny containers. Aspirin.
5. For my own kit, I also have prescription painkillers, Diamox, Flagyl, Cipro or Bactrim, and things of that nature.
Package it all into a durable and weatherproof container. It might be a red nylon zipper bag that opens up so that you can see everything at once. Mark the outside FIRST AID.
Once you get all done with that, you can take the advanced class in emergency surgery on the trail using your Swiss Army Knife. Been there. Done that.