Step 1:
The trays are folded out of 35x35 centimeter papers.
Prefold in half over these 2 lines.

Step 8:
Fold this edge upwards

Step 6:
Prefold over the left line and fold the 2 corners backwards.

Step 10:
Glue the edges together, see arrows. And fold over the prefolded line of step 3.

Step 9:
Fold the edges against each other, see arrows.

Step 5:
The valley-folds are prefolded lines.

Step 7:
Pull these 2 edges upwards

Step 3:
Fold 3 edges backwards and one edge to the front.

Step 2:
Prefold 3 edges to the centerline

Step 4:
The line on the right is a prefolded one.

Picture: Useful and decorative paper office models

Done!

I have a video tutorial on Facebook for folding the letter tray