Nowadays, most cans come with a lid similar to the ones you see on soda cans, allowing you to simply open the pop-top to get at the can's contents. However, there are still some cans that require a can opener in order to function. When there's no automatic can opener available to open cans, your next best bet is one found on a multitool.
Extend the can opener as far as it will go.
Align the outer edge of the can with the curved point at the bottom of the opener, such that the curved end is under the can's outer lip. Using the curved point and can's lip as leverage, stab the top edge of the can with the sharp end of the can opener.
Hold the can opener firmly while turning the can, continually stabbing the top edge of the can as you go.
Lift the can opener up slowly once you have gone full circle and remove the lid.