Manhole covers, used to seal access to plumbing, electrical and sewage systems, are heavy. Lifting one alone and by hand is a bad idea unless the diameter of the cover is less than two feet. Even small covers may be deceptively heavy. If the manhole belongs to a utility company or municipality, don't lift it at all. If it is on your property, be sure you have a good idea what lies beneath. Methane gas leaks might overpower you or exposed electrical lines electrocute you.
Clean off the top of the manhole cover. Debris that collects in the crack between the cover and its frame may be thick and impacted. Sweep first and then scrape with the claw end of a crowbar to loosen the debris. Do not smack the cover with a sledge hammer to dislodge debris; this can crack the cover.
Locate the hole in the manhole cover; it is usually in the centre. Insert the lifting key. Place a foot on either side of the manhole. If you can do this without squatting, you might be able to lift the cover. If not, you probably will need a mechanical lifter and another person.
Stand with your back straight and knees bent. Lift the cover just enough to clear the frame. Do not stand on the frame where the cover rested; it might be rotted and break away. Drag the cover away from the hole very slowly using the key. Do not lift the cover with your hands; if you drop it, it could crush your hand as it falls. Pull it away at a 45-degree angle; do not try to raise it completely off the ground.
Step away from the hole until you are certain no gas is escaping. Shine a flashlight into the hole to reveal the condition of the hole and its contents. Do not enter when you are alone. Put caution cones or gates around the hole.
A manhole lid that has no holes must be lifted with a powerful magnetised cover lifter that has a winch mechanism. This is professional equipment unavailable at the local hardware store.