Gather the tray parts, align the front and side rails. Glue together.
Be sure that the outside of the rails are even, or slightly overhanging the outside edge of the tray base. This will give your based miniatures the maximum amount of space to fit.
Each part has a clean side, and dirty side. The dirty side can be identified with the laser scorch marks spaced around the edge of the part. The laser cuts the parts with a slight taper, meaning the top clean side of the part has a wider cut width than the bottom dirty part. By assembling the tray with the dirty side of the rails upward, you will gain a little more room between the tray rails, and make fitting the bases much easier.