Use a Construction Rotary Laser Level

This article will help you to learn how to use a construction rotary laser level.

Steps

Purchase a laser level first. There are many different kinds to choose from out there and you might like to try either online or at a local hardware store. Choose from manual-leveling, to auto-leveling, and if you want to use one outside or indoors. If you plan to use one outside, you will need to buy a laser detector, some people call them receivers.
Compare all laser level accuracy at several distances indicated in their spec sheets.
Also you should consider if the laser level has horizontal, one or two inclined planes capability.
Set the laser up on a semi-level surface, a sawhorse, bucket, or if you have an aluminum or wooden tripod, that's the best surface. Set the laser level at an elevation that is higher than {{safesubst:#invoke:convert|convert}} or lower than {{safesubst:#invoke:convert|convert}} so that the laser ray do not coincide with the eyes of the staff.
Turn the laser on and let it get it level. It is recommended that you should purchase only lasers that auto-level when you turn them on. This will ensure greater accuracy and speed.
Begin measuring. Once you have a level line around your work area, or room, you can use that line as a reference, you can make marks on it, then come back and measure down from there, or measure down or up to your work while the laser is running.
Turn the laser off. Now you have a nice level reference line to hang kitchen cabinets, or install chair rail, siding, wallpaper, etc.

Don't knock your level over, or drop it.
Turn your level off, let it cool before putting it back in it's case since it can sweat.
Keep plenty of fresh batteries, and/or make sure you have an AC adapter or charger handy.
The photo above shows the level in use at the time the photo was taken. The workmen are using it to get their levels for the top of the sand bed / bottom of the concrete footing. The guy situated on the top will, from time to time, pass the staff (the white item leaning on the pile of sand) down to his mate in the trench. When the staff is at the right height, the receiver on the staff (the crosspiece just behind the tripod leg) will give a series of beeps.
Buy an auto-leveling level. Don't buy a manual leveling level.