I started the box by making the top and bottom frame and panel, mainly because this was new territory and I wanted to keep my options open. I figured that I could fit the rest of the box to the frame and panels if they ended up being slightly off in size.
I completely blew the first mortise by chopping right through the frame with my chisel. This was caused by a combination of laying out too deep a mortise and hitting the chisel too hard while chopping. Luckily I had some extra wood, So I redrew the mortises slightly less deep and hit the chisel with more care, especially when reaching the bottom of the mortise.
After making the frames, I measured and cut the panels to size. I bevelled the outside and rebated the inside of the panels.
I was quite pleased with the end result. The frames were square and the panels fit!