Nicely done. If I can offer some advice...
1. A 2x4 is neither 2, nor 4.
2. The lengths of any prepurchased lumber cannot be assured. Measure, and cut. This includes plywood.
3. Get a nice powertool either battery or cord operated with interchangeable parts. You'll need an impact driver (screwdriver) and a drill and a saw at the least.
4. Deck screws, not nails. Nails come out. Deck screws are easy to put in with your new impact driver.
5. Measure twice cut once. For those of us out there that are slightly idiotic with 3D... make that review the plan twice, measure twice, cut once. Hopefully.
6. If you screw up, try try again, or try to change your plan to match what you just did.
7. Think everything through. If you're going to cut something out, make sure whatever you're putting in there fits, moves like it should, etc. Walk through the steps before finalizing it.
8. Phone a friend. At the very least you have someone to laugh with, at the most, someone to bounce ideas off, hold stuff, fetch stuff, etc. If you're cutting plywood, it's also helpful to get one of those chalk line things that you can pull out and zip back up. One person holds each end where the measure mark is, then snap the line down and cut. Much easier than eyeballing it.