Before you even make your list of necessities, consider how the first aid kit will travel. A kit that sits under a picnic table or in the back of a car can be very complete since nobody has to carry it. By contrast, a first aid kit for backpacking will need to be small and light, with only the essentials.
If you need to use a smaller kit, consider multipurposing other items you bring. Your flashlight and pocketknife will be useful in first aid, but don't need to be part of the kit itself. For large burns and cuts, you can use a clean shirt rather than a bandage.
Decide what kind of container the first aid kit will go into. If your needs allow for a large size, fishing tackle or tool boxes make great containers for your first aid kit. They're durable, easy to open and subdivided into sections for organizing your gear. More portable first aid kits usually go best into a small, soft-sided zippered bag like a fanny pack.
A first aid kit requires material for treating cuts. Avoid tiny bandages: a cut that small won't really need a bandages. Do pack a reasonable number of standard size bandages, gauze and medical tape. Pack a tube of antibacterial ointment and a pair of tweezers to help remove debris or splinters from a cut.
If you plan to use this first aid kit out camping or backpacking, include equipment to help patch serious bleeding on the way to medical attention. This might include butterfly bandages and compresses. Consider packing a maxi pad or two to use as compresses.
Burns are common outdoor injuries. Pack a tube of burn cream to treat minor burns, and use the gauze to cover and avoid infection. If you have room, a portable chemical cold pack is a good replacement for the cold water bath that's usually recommended for first degree burns. Sunburn cream is another good item to include, if you have room.
For a break or sprain, you need splints and enough bandages to keep them stable. Pack at least one roll of compression bandage and one roll of athletic tape. The splints themselves can be cut off nearby brush, so pack a pocket knife. To reduce swelling, pack a portable chemical cold pack and travel packages of an anti-inflammatory.
Your first aid kit should also include a basic medical guide, flashlight and space blanket (a lightweight, extremely thin reflective blanket). Finally, consider the specific needs of people likely to come with you. Those with bee sting allergies should have a bee sting kit. Diabetics will definitely require some hard candies, and might need an insulin rig.
