These things suck. Here's how to make a better one.
Mostly what they give you is a bunch of bandages, and bandages suck for real first aid. They're finicky to use, and you never have enough of the right size. Instead of these, I bring 4"x4" gauze pads, a small pair of sharp scissors, and a roll of good quality paper bandage tape. With these, I can quickly cut as many bandages as I need in any size and apply them securely exactly where they need to be, with the pressure pushing in the direction to close up the wound, all with one hand. I also take some saran wrap and duct tape too for waterproofing the area, as I find this works much better than using "waterproof" bandage tape. I find waterproof bandages always tend to let moisture wick through to the wound, which gets it irritated.
In addition to this I bring a good quality cloth elastic bandage wrap to apply controlled pressure to joints etc. The most important part of these wraps is the steel hooks that hold them tight. On cheap ones, the hooks are stamped out of sheet metal and they always fail on me so I end up having to use safety pins (bring some of these too btw). So I would get the good one up front instead of wasting time in the field.
And that's it, basically. Yes there are other things that are good to bring along like alcohol swabs or a tourniquet, but in practice I've never actually needed them. The gauze pads and elastic bandages have saved my ass multiple times though. So this kit is just based on my own needs. I don't claim to be any kind of expert on this, so you'll probably need to change it up as you find necessary. Just wanted to give people some pointers who might not be experienced yet.