First, lift one large flap up 90° (orange arrow) then lift the side of the box up 90° (blue arrow). Repeat this on the opposite side and the large flaps should then overlap and can be glued in place.
Then, repeat for the other side.
This forms the box part of the project.
Next, for the clamshell cover.
Fold on small flap 90° (orange arrow), then fold the side up 90° (blue arrow) and then fold the lid over 90° (purple arrow). You should now be in a position to glue the small flap to the section at the end of the purple arrow.
Repeat this for the other three lid sections at each corner.
You should now end up with something like this … (without the decoration).
Speaking of decoration, fancy seeing how I did that bit…?