Prefer step-by-step instructions? These should help you get a great result:
1) Clear off a clean, flat work surface. Ideally, the surface should be larger than the size of your unassembled box. For best results, do not layer the boxes while folding as this can damage the boxes underneath and also result in bad creases in the box you're working on.
2) Examine the scores (pre-fold lines) on your unassembled box. One-by-one, fold over each of the scores on all parts of the box completely towards the interior, then release it back to flat on the table, interior facing up.
In two areas there are scores that are very close together. It is essential that you take care to fold each of the creases separately where they occur on the sidewalls. Otherwise, the box will not correctly lock together.
3) Fold up the front and back flaps, positioning their tuck flaps along the side of the box as much as possible. Brace the back in place lightly as you complete the next step.
4) Bring up the side walls of your box by folding the left and right side flaps over the tuck flaps, then snap them into place at the same moment. This ensures a very symmetrical, even structure where the lock tabs are equally snug.
The key visual to know it's right is when the tops of the side walls are flat, not slanted or peaked.
5) You can now add your contents, close the lid and tuck the side flaps into the space left in the side walls to lock it shut.
The result should be a nicely sturdy, locked box.