As I seem addicted to making Box Cards and that UKPapercrafters will be doing them for our SWAP next month.
Here's a little tutorial.
I have only made my boxes up from A5 card so far, they fit snugly into C6 envelopes.
Firstly, from the shorter side of your A5 card score at 2", 4", 6" and 8" which will leave a little flap to make up your box with.
From the longer side score in at 2 3/4" (leave the section next to the flap unscored as this makes the upright back of the box - although if you forget you can always glue solid card on both sides afterwards.)
Cut into your initial score lines up to this new score line and snip off the top piece of the flap at an angle.
You can round the corners if you like, I quite like them rounded.
It's best to decorate the outside of your box BEFORE gluing it into a box shape.
Inside the box top opening are glued two 1/2" x 3" strips of card scored 1/2" from each end and folded into a "Z" shape. It you use wet glue and squash your box flat whilst the glue dries, you'll know it will fold flat afterwards.
Choose which way you wish your box to fold flat and stick with this as it will dictate how much you can extend the internal items over the edges with them still being within the folded shape to fit in the envelope!
You can now add all your items