One of the tools I always suggest for anyone using hand tools is a shooting board. In its most basic form it lets you hand plane wood to perfect square, and with a few more steps you can add things like a bench hook for surface planing, and any number of options for cutting/planing mitres.
My shooting board is really ugly and in need of repair, but it’s the basic form of one with a bench hook. The lip on the bottom pushes against the front of your work bench so you can use the top’s lip as a second hand. The bottom of mine is covered in leather scraps to make things a bit quieter.
Construction needs a few perfect 90 degree angles. Plywood is great because you can buy it with those angles (I got mine at a craft store), and it won’t lose its shape over time. As you see above mine are no longer at 90 degrees, but to fix that I just need to re-plane those areas so it’s a quick and easy repair. A lot of people recommend a slight angle on the plane side, and you can see why on the right picture. The top tends to go first, and the angle will extend its life.