Shift the bike into the smallest rear cog and smallest front chainring if it has gears
Loosen the rear axle nuts or quick-release skewer
Pull the rear wheel straight back in the dropouts to increase chain tension
Keep the wheel centered in the frame while pulling back
Check chain tension by pressing the chain up and down at the midpoint
Leave slight up-and-down play in the chain, not overly tight
Tighten the axle nuts or quick-release skewer while holding the wheel in place
Spin the pedals to make sure the chain runs smoothly
Recheck wheel alignment and chain tension
If the bike has a derailleur, adjust the derailleur or replace the chain instead of tightening it manually
If the bike has a single-speed with horizontal dropouts, use the dropout position to set tension
If the bike has a chain tensioner, adjust the tensioner according to its design
If the chain is worn or stretched, replace it
If the bike has a master link, inspect it after adjusting
Test ride the bike and confirm the chain does not slip or feel too tight