Begin with a piece of cardstock cut to 5 1/2" x 8 1/2". Score at 1/2", 2 1/2", 4 1/2", and 6 1/2". Then score along the other edge, beginning at the end without the 1/2" score line, score at 3" but end the score line after three of the scored sections as shown.
I've written the measurements on the template. Please note that the red dotted lines mean cutting lines. The remainder of the lines are scoring lines. You will note the absence of a line on the last section, right by the tab. This section remains unfolded while the rest fold over as shown below.
This is how the paper looks folded. You will also require one additional strip of cardstock measuring 1" x 5". Cut this in half. The two pieces will be 1" wide and 2 1/2" long. Each one is scored at 1/4" and 2 1/4" and then the ends are folded in a "Z".
Put double sided tape along the tab for the box and also along each of the 1/4" tabs on the small pieces.
This is how the box will look assembled. There is no bottom to the box. The inside dividers (small "Z" folds) are approximately equally spaced.
And after that it's all a matter of decoration. Be careful not to have too much of an overhang at the top or sides or it will make it difficult to fit the collapsed card into the envelope.
If you run into any difficulties, just drop me a line and I'll try to help you through it. Have fun.
NOTE: You can also make this box out of a piece of 5" x 8 1/2" cardstock. This will allow you to place some of your embellishments a bit higher out of the card and still allow the folded card to fit into the envelope. In this case, follow all the directions above, but score at 2 3/4" instead of the 3". This will give you a base that is proportionate to the top.