So here are the instructions - for any size!
1. take an A4 sheet of cardstock
2. place it landscape on a scoring board and score at (this is where you decide the size of your box! Lets go for the largest we can from an A4 sheet).. score at 7cm, 14 cm, 21cm and 28cm
3. Turn to portrait and score at 7cm
4. Use a cutter and, portrait with the 7cm (in 3 above) on your left - cut at 14cm
5. Save the strip left - you need it!!
For the rest here's a pic! (click on it so you can see it all!)