Hey, I'm going to show you how to make a box (about the right size for a gift card or jewelry) out of a birthday/christmas or any type of greeting card.
I learned this from a lady from a charity organization who makes these for gifts and such.

Step 1: "Supplies!"

Okay, you'll need:
A ruler with cm measurements.
Tape or glue.
A card of some type.
Scissors.
And a pencil.

Step 2: Cut the Card.

Okay, take your card, keep it closed, and cut along the fold. Simple! Try to remove as little material as possible.

Step 3: Mark Fold Lines.

We'll need the back half of the card for this. Before you mark any lines, cut a very small amount (a couple of millimeters) off one end. Done that? Good. Now make two lines along the long ends 2 cm from the edge (see second picture). Okay, now make one line 2 cm from the edge on one of the short ends (see third picture) then make a second one 2 cm down from the first, (again, see third picture) do this on both ends.

Step 4: Snip Snip.

Okay, now cut along the line shown in picture 1 down to the line that goes all the way across the width of the card. Do that to all four corners (pictures 2 and 3). Simple!

Step 5: I Fold...

Okay, take your ruler, and line it up with one of the long lines, then press the ruler down and bring the long side up, see second photo. Do this on both sides. (see third photo). Then fold all the rectangular tabs in, make sure the edges all line up. (see fourth photo). Then, line your ruler up with the first of the shorter lines on one end, and fold like the long sides. (See fifth pic). Then do the second line, (see sixth pic) you should have two folds, now do the other end.

Step 6: Assemble the Bottom Part of Box.

Okay, time to fold the box together.
Bring the long sides up, and fold the tabs into the center, then bring one end up and fold the lip over the tabs. (see second pic). Repeat on other end. Then tape or glue the lips down. Your down with the bottom!