You can't. Barbed wire is stretched by pulling it through a series of rollers, which cause the barbs to open and stretch the wire. If you don't have access to a set of rollers, you can try using a pipe or other cylindrical object to duplicate their function. However, it's important that you use something that is smooth so as not to damage the wire.

There are a few ways to stretch barbed wire without a stretcher. One way is to use a piece of wood or metal as a lever. Place the object under one end of the wire and apply pressure to the other end until the wire stretches. You can also use a rope or chain as leverage. Place one end of the rope or chain under the wire and pull on the other end until it stretches. Finally, you can use your body weight to stretch the wire by lying or standing on it.