First, measure the area you need to know the depth and width of the closet so you know how big you can make the shelf.
You'll be using the one by two boards as cleats so cut one of the boards to the width of the closet and the other two to the depth minus the width of your first one by two.
Once you have the boards cut to size, grab a torpedo level. Hold the board in place in the closet with the level set on top of it; adjust until your cleat is level and even. Then use a nail gun to secure the board in place. Make sure that you hit studs wherever possible.
A tip, if you don't have a nail gun, use some hot glue will let you get the board up and level and then dry quickly enough that it will hold there while you use a hammer and nails to secure it in place.
Repeat the leveling and securing for the other two sides.
Once you've finished securing the cleats all you have to do is lay the board in place.
If you'd like you can paint it out to match the closet! see how easy it was to add that extra storage. I'm Mrs. Fixit and it's just that simple!