Loosen the axle nut & back off the adjuster bolts at the rear of the swingarm. Step on the drive chain to move it up. Turn the adjuster bolts to tension the chain, checking at several points by rotating the wheel. Some variation in tension as the wheel is turned is normal but if excessive chain and/or sprockets are worn. If in doubt, replace them all. Set the adjusters equally by seeing that each side has the same distance according the the marks on the adjuster plates & the swingarm. Use a long straight pole, string or eyeball to check that the rear tire is in alignment with the front, allowing for the probable narrower front. If alignment is good use the index marks on the swing arm in the future unless damage occurs. Tighten the axle nut & repace the cotter pin & tighten up the locknuts on the adjuster bolts or they will fall out. Grasp the chain at the rear of the rear sprocket & pull away to guage wear. If 1/2 roller diameter or more is noted with the chain tensioned the chain is worn out. Look for "hooking" of the sprockets on the driving side of the teeth. Any visible wear is excessive & the sprocket is worn out. If mechanical linkage, adjust the brake for minimum pedal movement wihout drag, then adjust the brake switch to work properly. Lube the chain.
Ride Safe & enjoy your bike, Don (Psycle) Madden