I do apologise before we start that some of the photos are not very clear, my camera did not want to focus on this colour cardstock, hopefully though you do get the idea and will enjoy making this box for yourself.
Okay you are going to need a Score board (or something you use for scoring, I use to use just a trimmer and score tool)
51/4"x 91/2" cardstock of your choice
Take your 5 1/4"x 9 1/2" cardstock and place it on your score board so that the long edge is at the top and score at the following marks:
1", 4 1/4", 5 1/4" and 8 1/2"
Next turn your cardstock 1 quarter turn so now the short edge is at the top of your score board and score and the following marks:
1" and 4 1/4"
Fold in each of the score lines then cut and trim out little triangles so that your cardstock now looks like this.
Turn your cardstock over and add adhesive to each of the small tabs as shown here.
Fold up your box and adhere the tabs so they are on the inside of your box
Your box is now ready to be filled either with 3"x3" cards, or a small gift of some sort. Lastly all you need to do is decorate the top of your box.