I decided to remove material to create the ledge for the plywood to sit on later. You could chose to add material instead though which is fine.

I cut a groove out of the 4 rail pieces (Red & Green). The section I remove is 45mm deep and 19mm wide.

The joinery for these pieces is a simple half lap joint. It may be a little awkward to get your head around because of the groove we cut but take your time and make sure its laid out correctly before cutting.

The short green rails sit on top of the long red rails. So using a hand saw, I remove the 38mm cube from the bottom

The long Red rails go under the short green rails so I remove the 19mm piece from the top.

Its very quick and easy to cut these pieces by hand. They should go together nicely.

Hopefully my pictures will help to show what needs to be removed for each piece.