Bicycles need to use chain tensioners also known as "chain tugs" in the absence of a derailleur, which make up for the excess room on a chain on a single-speed cycle. Chain tensioners attach to the axle on the rear portion of the cycle, and when you secure them in the correct manner, slide the wheel away from the middle, which removes the extra room from the drive chain.
Place your bike in an upside-down position, and put it on an area that does not scratch easily, like a towel.
Take off the nuts of your bicycle axle on the back with your adjustable wrench.
Thread the hole on your tensioner over the exterior part of your axle. Do this for each end of your axle.
Place both chain tensioners in such a way that their adjustable exterior parts lie flat against the rear portion of your cycle. This allows your tensioners to sit parallel to the floor.
Secure the bolt that lies on the rear portion of your tensioners with your adjustable wrench. Repeat this process until you have no more slack left on your bicycle chain.
Put back your bike's axle nuts by hand. Then, secure them completely with your adjustable wrench.