So first of all I cut the flaps off my cardboard box and measured the length, width and height. This is needed for when you start measuring and sewing your inlay.
For your inlay, measure so that the width of your fabric is at least twice the length of the height of your box, and so that the length of your fabric is the combined measurements of the width and length of your box.
Pin the fabric in place and sew along both short edges and one of the long edges, leaving a long open end to what should look like a flat ‘pouch’. The correct side of the fabric should be on the inside of the pouch.
Hopefully my pics help with this part!
To make the box shape, you now need to sew in some corners. Open up the pouch and flatten the two sewn ends down to create right angled ends and sew straight across each corner.
The distance between the red lines, as shown in my pics, should equal the length of your box. The distance from one of the corners to it’s nearest red line should equal half the width of your box. This means that the actual length of your red lines should measure the same as the full width of your box.
I hope this step doesn’t sound complicated because it’s actually such an easy way to create a box inlay that once you understand it, you’ll be able to use it for so many other useful sewing tricks, including bag making.