Lacking a derailleur to take up slack in the chain, single-speed bikes, such as BMX, require the use of tensioners, sometimes called "chain tugs." BMX chain tensioners come in a set of two, one for either side of the wheel. These devices attach to the rear axle and, when tightened properly, pull the wheel away from the centre of the bike, in turn, removing excess slack from the drive chain.
Turn the bike upside down and place it on a non-abrasive surface, such as a towel. Turning the bike upside down will allow you to keep the bike upright while working on it. The towel will keep the bike's handlebars from potentially getting scratched.
Use an adjustable wrench or proper-sized socket and socket wrench, and remove each of the axle nuts from the rear axle. The nuts turn counterclockwise to loosen. There will be one nut on each end of the axle.
Thread the opening on the tensioner over the end of the axle. Do this for both ends of the axle. There will be one tensioner for each end.
Pivot each tensioner so that its adjustable end is flush against the back of the bike frame. When properly installed, each tensioner should be parallel to the ground.
Use the adjustable (or socket) wrench and tighten the bolt at the back end of each tensioner. Continue to tighten the bolts until excess slack is removed from the chain. There should be no danger of the chain slipping off the front chain ring when pedalling.
Reinstall each axle nut, threading the nuts on by hand until they are seated. Once seated, tighten the nuts fully with the adjustable (or socket) wrench.