I select an existing first aid kit to purchase, for the packaging material, for including materials I anticipate possibly needing to use, and for having a nice instruction sheet or instruction booklet, if possible.

REI has a well thought out REI Day Pack First Aid Kit.

Adventure Medical has a number of useful 1st aid items in kits for backpacking.

The Adventure Medical kit for the backpack I like is the Ultralight & Watertight .7 or Adventure First Aid .5

Kayakers may want to consider the Lifeline Waterproof First Aid Kit.

May I suggest you make the first aid kit your own?

Begin by adding things you may need. If there are special needs, these go in the first aid kit.

The remainder of the kit is more intangible. What sort of minor or potentially incapacitating and major injuries could be encountered? What cannot be improvised?

Blister care, wound management, burns on fingers and hands, ankle sprain, bee stings, poison ivy or poison oak, dental problems, syrup of ipecac, oral electrolyte replacement, antihistamine, antidiarrheal, activated charcoal may be needed.

If you want something in the first aid kit, do learn the proper use.

My present first aid kit includes: individually wrapped sterile gauze flats, non-stick gauze flats, Xeroform 1x4 "finger-size" burn bandages, surgical tape, self stick stretchy roller gauze bandage, small butterfly bandages, knuckle bandages, fingertip bandages, moleskin, folding scissors, second skin moist pad, Tylenol, aspirin, Extra-Strength Excedrin, Benadryl, tiny kit bottle calamine lotion, triple antibiotic ointment, benzalkonium chloride antiseptic towlettes, benzoin tincture ampule, Leukotape P, Kinesio athletic tape, packet of sterile water for irrigating a wound, 3% USP hydrogen peroxide, CPR microshield, surgical gloves.

I have had laminated first-aid instruction book pages in my first aid kit for many years.