First you need images of the flags of the countries you want to show.
Copy two images of the same kind one besides the other into a word document.
(You can make more than one at a single blow)
There must be an edge on both sides.
Print the document an fold the paper, that your flag gets two coloured sides.
Cut the pictures and glue the inner side.
Roll the white edge around a wooden stick and fix it with glue there.
At last put it into a styrofoam block.
Example