Textsheets can be downloaded for free from this site. English texts are available as well as pictures only. Just follow the link "TEKSTSHEETS" and then browse through the options on the bottem of the screen.

Cut a circle out of a textsheet. Leave a margin of around 5mm.
Cut a circle out of a card in the desired size. Be careful not to damage pictures on the front!
Put small pieces of two sided adhesive tape around the opening on the backside of the card.
Put the sheet in position. Be aware of front and backside!
That's it!