First of all, choose a simple design, I'm no card maker and I managed to make mine fairly quickly. I made these cards and the box in an afternoon.
To make the box you need to follow this template.
To get the dimensions of the square/rectangle in the middle (A) you need to measure the envelope that your cards/notelets will be going in.
For the base you need to add 2mm on each side (so if your envelope measures 140mm x 140mm then you want to measure out 144mm x 144mm; if your envelope is 215mm x 85mm then you want to measure out 219mm x 89mm).
For the lid you need to add 4mm on each side (so 148mm x 148mm; 223mm x 92mm).
To get the dimensions for the depth of the box (B) you need to put your 6 cards and envelopes in one stack and measure the depth of them. Bear in mind that they will flatten, if you allow too much depth they will float around too much in the box.
For the base measure the depth as above and allow a couple of extra mm just to be on the safe side.
For the lid, depending on how deep the box is you can allow less (for instance a 50mm deep box would only need a 20mm deep lid) but if the box is fairly shallow anyway then to work with much less than 20mm for the lid becomes fiddly so it can be the same depth as the base.
Measurement (C) will be folded in to reinforce (B) so it will need to be 2mm less than measurement (B).
The tab size is not important, if it looks similar to the diagram then fine.
One thing to note about the section (C): as this folds inside the box it is better if it is slightly tapered so it fits easier.
Once you have cut out your base it should look like this.
Score and fold along the lines of the template and fold in and glue the tabs like this:
Then fold over and securely glue the outer parts of the template, these will hide the tabs and reinforce the sides of the box.
For the lid, measure and cut an aperture approximately 10-20mm in from the edges of the centre square/rectangle.
Cut a piece of acetate that fits snugly in the box lid and glue.
Fold in and glue the tabs and outer section as before.
Your box will look something like this
For a final touch add a label to the lid and print or write the contents on it.
And you're done!