Start by cutting a square 13 x 13cm from the front of the card and a square 12 x 12cm from the back; draw diagonal lines from corner to corner to find the centre of each one then follow the folding and cutting as shown in the photos below.
I have drawn over the cut lines so they are easier to see. No glue required. The top and bottom should fit snuggly together.
It looks more fiddly than it really is as you will hopefully find out when you
have a go!