Make these Cuts:
1 piece at 6 feet x 2 feet 11 1/2 inches (back)
2 pieces at 6 feet x 11 7/8 inches (sides)
6 pieces at 2 feet 10 inches x 11 7/8 inches (shelves)
1 piece at 2 feet 10 inches x 1 3/4 inches (bottom shelf support)

Sand all surfaces (if you choose to edgeband as well, do it now).

Attach one shelf to the top of the two sides. Make certain that you are flush with top, front and back. (It's easier if you lay the bookshelf down on its face throughout the entire construction process.)

Attach the bottom shelf support to the bottom of the sides so that it stands 1 3/4 inches tall (if the bookshelf was standing up). Check that it is flush with the bottom and front.

Attach the bottom shelf. Lay its front edge on top of the support. Attach to both the support and the sides. Make sure there is a space of 1 3/4 inches between the back of the shelf and the bottom of the sides.

Attach the shelves. To space them evenly you will need a space of
13 3/4 inches between each shelf. But it's your project. You can make all the shelves adjustable if you want or put them in any fixed configuration. Make sure you include the 3/4 thickness of the shelf when you are measuring.

Attach the back. Lay the back on top of your face down bookshelf. Make sure it is square with all edged. Attach it to both the sides and the back for exra support.

Paint or stain to your liking. Or leave it unfinished for a more modern look.