Last year I helped organize and prepare my Venture (scout) company to go on a 200km cross country bike trip along the Kettle Valley Railway. Once everyone (10+ people) had a suitable bike and panniers (saddle bags), and cooking and cleaning equipment the only things left was to maintain about 8 bikes that were bought from local classifieds and un-tested. Buying 20 (two each for good measure) patch kits would not have been terribly expensive, but saving any cost is always good. Besides all the things that are in a patch kit were laying around my house.
Step 1: Materials and Tools
Most of the things you need can be bought at a hardware store.
Contact Cement (500ml or less)
Sandwich bags (1-2 per kit)
Sand Paper (1 square inch per kit) - optional
Old Inner tube (1 will do)
Spoon (disposable hopefully)
hammer and/or tin opener (or COLD chisel) to undo and do up the contact cement can.
Be careful when opening the can and read and follow the warning label on the contact cement.
Note if you're not using sand paper use sand from the location you're in instead, if you're location doesn't have any sand or gritty substances make sure to pack the piece of sand paper.
Step 2: Cut to Size
Start by cutting the inner tube and sand paper into smaller sizes. The Inner tube should be cut into patch like shapes, circles or ovals not squares and rectangles. Also when cutting the inner tube make sure to cut the final patch with the scissors on an angle so that the patch has beveled edges, slanting up to the center of the patch. The sand paper can be cut to whatever size is convenient, if you're using sand paper.