As far as useful tools in a jam go, it's tough to beat the general practicality of a knife. Whether it's marking a trail, field dressing an animal, or defending yourself, a sharp piece of steel on your hip can solve a number of problems you may face in a survival situation; which is exactly why so many people maintain a good quality knife in their EDC (Everyday Carry) loadout. But what if you find yourself stuck in a long-term survival situation without ready access to a knife?
You could go on without one, or you could make one, using nothing but a few common hand tools and some scrap metal.
The first thing you need to do is find yourself a suitable piece of metal. While you can usually get a sense of the sort of metal you're working with with a visual inspection (stainless steel holds a shine while carbon steel will brown or rust, for instance), your top priority is finding a sturdy piece of metal that's somewhat close to the size and shape of a knife. The closer it starts in size, the easier a day you'll have. For a good survival knife that fits well in my hand, I usually prefer a piece of metal that's somewhere between 10 and 12 inches long, less than a half inch thick, and 1.5 to 2 inches wide, but it may take some work to cut your piece into those dimensions.
Low carbon scrap steel is soft and doesn't make for excellent knives, but in a pinch, even a rusting blade that needs sharpening is better than no blade at all.
There's no getting around the need to have a hack saw when working with metal. In fact, if the scrap metal you locate is too long, it's the only tool you can't do without. Beyond that, all you really need is a metal file or access to plenty of sidewalks or blacktop. Any of the three will do.
Cut the steel into the general shape of a blade
Use your hacksaw to cut your scrap steel until it meets your general length and width requirements. The harder the steel (based on carbon levels and if it's been treated) the harder the cutting will be. Be patient and careful not to hurt yourself. If you need to make this knife, chances are good no ambulances will be coming if you suffer a nasty gash.
Grind, grind, grind
If you have a metal file, hold the knife in one hand while carefully using the file to shape the profile of the blade. Be careful, if you have access to a vice, put the blade in it while you work. If not, finding a pair of work gloves can help keep the skin on your fingers.
If you don't have a metal file, you're not out of luck. Sidewalks and black top are very abrasive surfaces, and you can whittle away at the metal edge of your blade using either with enough patience and care. This is a great way to shave your knuckles, and you will ruin your driveway, but I've managed to fashion a workable blade or two using this method.
Making a handle
Depending on the tools you have on hand, there are probably rough metal splinters hanging off the edges of your knife and once your hand gets sweaty (or bloody) keeping a grip will be impossible. Fortunately, there are lots of materials that make for decent handles.
Lots of military guys are familiar with making things out of paracord, and knife handles are no exception. Leather belts, rope, and duct tape are all excellent knife handle materials. Wrap the material around the handle of the knife as tightly as you can, overlapping it by however many layers as necessary to make the handle a comfortable girth.
Then get your ass out of dodge with your new prison-shank in hand.