How do you replace the freeze plug in a 1991 Ford Ranger?

short answer: take it to a shop. it may cost a couple of hundred dollars, but you'll have some sort of guarantee/warranty if it leaks or something goes wrong.

longer answer: crawl under the truck, locate the freeze plugs along the sides of the engine. they look like little cups pressed into the block. (a trip to the local library is helpful if they have an automotive repair section.) if the plug is still there, knock one side with a flat blade screwdriver so that it "flips" sideways in the hole, then grab with pliers and pull it out. replace with a new one by finding a socket similar in size and use that to tap the new one into place. (do NOT go too far, otherwise it'll be an automatic trip to a shop and cost several hundred more.) start engine and let it run until normal operating temperature is reached and check for leaks.