Alright, I've taken measurements, researched what kind of wood I'd need to start, and hit my first road block: how do I go about making a box?

I've been looking around for some tutorials and such, but no luck for me! I'm interested in making a 4'x7.25'x1.25' box. One half of the box will need a deeper excavation than the other side. For those interested in this project for any reason, it's the only other topic created by me.
If someone wants to give me a walkthrough, I'd appreciate it, though I'll take any help anyone will offer. Thanks!