I always tend to draw things out to see what I am doing first.
I am very much a visual learner, hope the photos help.
All measurements were based on 1/2" plywood!
Bottom: 20" x 12"
Sides (two of these): 6" x 20" x 10" x 10"
Back: 10" x 11"
Front: 6" x 11"
Top: 12" x 10"
Now for the assembly. Once you have all of your pieces cut, assembly is a snap.
Apply the sides so that they sit on the bottom. For all the cuts to work this is imperative. The sides so not go on the side of the bottom, but on the bottom. We have an air gun, so we stapled it. You can use a handle stapler, small nails, or small screws.
Once both sides are on, then you can move on to the front.
The front is put between the to small ends of the sides and on top of the bottom. I found this provides the best stability.
Then you can move on to the back. It also sides between the sides and on top of the bottom.
Then the top goes on. Be sure you have this on the right way. It should cover both sides and be equal in length to match the top of the sides from front to back.
Now you have a nest box. There are a few things I found I would change. Well, one really. For summer kindling I would put a wire bottom on instead of a wooden one. This way, the baby's (kits) feces and urine will go through. However, for the winter time here, I found the best for us was the solid bottom. With the straw in the bottom it will keep the bottom dry of feces and urine from the babies and keeps them warm enough.
My does look tiny in their box, but sine I breed in late winter, it is just fine as the amount of straw and fur in the boxes is enough to keep kits warm and allow mom in and out with out landing on the kits.