Hello and welcome!
Sometimes you need to make a box to fit a particular item. If you are going to make a box with a fitted lid, you measure the length, width and height of the object and then you need a piece of card that is Height+Length+Height by Height+Width+Height in size.
Your score lines need to be at the Height measurement on all 4 sides.
To make the lid, you either need to cut the card a tiny bit bigger or, and this is the method I prefer, cut it the same size but use a shim to adjust the position of the score lines (watch the tutorial to see what I mean!)
It's reasonably easy to calculate the dimensions if your item is a nice, regular shop, but what if it's irregular, like my polymer clay dragon here (based on the work of Christi Friesen who makes the most AMAZING polymer clay creations and writes wonderful books showing you how to make your own!)?
I'll show you how I do it in the tutorial below!