You can have someone hold the shelf for you while you use a level to double check the evenness if desired.
Hanging shelves on an uneven wall.
For example using an ordinary standard and bracket shelving system would push the shelf itself off the wall by about 1 2.
Use liquid nails to attach the front and optional sides.
With some setups the shelf rests on top of the brackets.
Or just using a bracket that would allow you to move the shelf away from the wall slightly.
Place the shelf frame with the bottom but no top where you want it on the wall and screw it into every stud that it touches.
Step 4 cut along the pencil line using a table saw.
Position the shelf on the wall.
Hang the shelf on the mounting brackets.
If you want the line to stay invisible place the top of the shelf on the line.
You can use the nail gun to hold them in place while they dry.
Adjust the level until the bubble is centered and mark the keyhole locations on the wall.
Hold the level against the wall at the height you want the shelf.
Then install anchors or drive the screws into the studs and hang the shelf.
Pick up the shelf with the fasteners attached and line it up on the wall.
Fit a floating shelf by pushing it onto the bracket instead.
Drag the pencil along the wall keeping the shaft flat to the wall and allowing the tip to leave a line on the shelf.
Use the drawn line to help you ensure the shelf stays even.
Attach the plywood top to the frame.
Then check the shelf to make sure it s sturdy and ready to bear the weight of whatever you put on top of it.