The length of the chain can only be adjusted in increments of 1 inch(link). On a derailleur bike a spring loaded arm holds the chain in tension. If the bike has only one position for the chain the distance between the cog-wheel connected to the pedals that pulls the chain, and the cog wheel on the rear wheel that the chain pulls must be exactly right.
Do one nut up finger tight. Carefully so as not to disturb the rear wheel switch hands and do the other nut up with your fingers.
Half tighten the nuts with a spanner. It’s usually a 15mm
If the bike has a fixed gear pick it up so the rear wheel comes off the ground and spin the pedals. If the bike has a freewheel you can do this by spinning the pedals backwards. The chain needs to be as tight as possible without creating friction that will waste energy and wear the chain and cogs prematurely.
Walk The rear Wheel
To adjust the rear wheel’s position loosen one side and push the rim between the chain stays to one side to move the loose end of the axle a short distance back or forward. Re-tighten the nut. Loosen the other side and ease the rim in the other direction so the rear wheel is – once again – straight in the frame.