Summarize:

The easiest way to make the top fit over the bottom is to make sure both halves of the card are the same size, so you may have to trim a little. You will use the colorful front of the card as the box lid. This may depend on the design on the front of the card, or on how big the item is that you want to enclose. This demonstration uses 1 inch (25mm) for the top, and 1 and 1/16 (27mm) for the bottom. Add a little to the measurement for the bottom, so the top will fit over it. You can mark just the corners, making a square so you can see where to cut.  Leave one side of the box uncut to make a tab.  Do this for all four corners of the card.
Gather your supplies and tools, listed below. Cut the card in half along the fold. Decide how tall you want your box to be. Draw a square in each corner of the top and bottom of the card, the size of your measurements. Score the card all the way along all four sides, using the squares to give you the distance from the edge. Cut along the score line from the edge to the next, perpendicular score line in each corner, but do so for only one side of the square you drew. Fold the sides up, and fold the tabs in. Glue each tab and paper clip it into place until it dries. Repeat the marking and scoring for the other half of the box. When your box is dry, place your gift or other small objects inside.