Where the two lines intersect is the centre of the card.
Do the same for the long sides.
In this photo I have drawn a line along the fold to indicate where to cut the card. I don't normally draw these in but just follow the fold.
Cut along the fold, stopping when you reach the perpendicular fold. (These are my husband's hands in this photo.)
At this point you can choose to erase the pencil mark, or not, as you prefer.
Fold the long sides up and overlap the little flaps as shown.
Repeat the process with the back of the card to make the bottom of the box.
Box is finished.
Here are a few that I have made. One year I made 120 boxes and filled them with lollies, stickers and little treats. I was a volunteer Scripture teacher and I made one for each of my students. A friend of mine once did a Christmas present swap with her family and the present had to fit in the box this size. I have a few around the place for special buttons etc. Cards with a centered design work really well.
I hope the instructions and photos are clear enough for you to make your own if you didn't already know how.