Measure your space. Measure the width (narrowest side) first and the length (i.e. 5x12). Take this measurement with you to the store.
Purchase your carpet runner. Most carpet runners are between 2.5 feet and 3 feet wide. They vary in length, usually from 6 to 20 feet, and sometimes longer. Decide how much of the floor area you want covered and how much floor space you want visible. Take into consideration any furniture and its placement near the runner. If you are laying your runner on top of carpet, you do not need to buy one with non-slip backing. It the runner will sit on bare floors such as hardwoods, you'll want to consider one with non-stick backing.
Purchase a runner pad or carpet tape. If the runner you purchased does not have a non-slip backing and will be placed on a slippery surface like hardwood, you should buy a runner pad or carpet tape. The pad should be about an inch smaller than the carpet runner's width and length. Keep in mind that carpet tape may cause damage to your floor if you later decide to remove the rug.
Lay your carpet runner on the floor. If necessary, tape the carpet tape along the edges of the runner or lay your pad down in your space. Place the carpet runner on top and enjoy.