Print the pattern and enlarge if you wish till A4 size. Copy the pattern with carbon paper on the cardboard and cut out the grey parts carefully. Be sure the letters will be stuck to the window. Paste tissue paper on the back.
You might prefer to let children draw their own letters. If you let them, give them the exact sizes of the window and tell them to be sure the letters have to be stuck to the window on at least every edge. This can be done with pencil and paper, but also on computer (Wordart).