These cabinets aren't very tall, so you'll want to lift them off the ground so they look taller and fill the wall space a little better. You also need something sturdy to drill them into so they don't move around. So the base is key.
Build two long skinny rectangles from 2 x 4's that will run along the edge where your floor was cut out (another good reason to cut out your floor — guideline!) A nail gun is the fastest way to assemble these. Set them in place end-to-end and running from one wall to the other, and check that they fit. If they do, nail in a series of short 2 x 4's running down the length of the cages. The smaller 2 x 4's should be placed inside the rectangle frames.
Then, screw a long 2 x 4 to the studs of your wall. The cage you just built will support the front of the bookcases, and the suspended 2 x 4 will support the back of them. (I know you have no idea what I'm saying. Just look at the pictures for a bit and hopefully it will make sense)!
Check everything with a level. Use shims to level it out if needed. Once it's all good, screw the cage to the floor.
