With bike off and in Neutral and on sidestand (Never check chain slack while bike is running):
Check chain slack on the bottom run midway between sprockets. It should be 5/8" - 1". If it is more than that you risk damage to chain and/or frame.
Loosen the axel nut (22mm and 17mm wrenches)
Loosen the lock nut on each adjuster (10mm wrench)
Loosen the break adjuster nut off 5 or 6 turns.(If not the will tighten up as the the rear axle is slid back.Then you will be wondering why the rear wheel will no longer spin.
Tighten the adjusting nuts until the proper chain slack is attained (Slowly, a little on each side to keep it even)
Make sure the chain stretch indicators are not out of tolerance. They should be aligned with the rear edges of the axle slots and not any further. More than that and you need a new chain. Also, if the chain is unevenly stretched (tight in one spot, loose on another) you need a new chain (and possibly sprockets)
Tighten the lock nuts,
Tighten the rear axle to 65fp or 88 N-m
Recheck chain slack
Clean and Lubricate chain
If chain slack and tension is good. Readjust the rear break back up.
You should inspect the chain slack after about 500 miles (or before each ride).