Cut the borders of the of the long boards with the circular saw (table or hand). As it is very difficult to get the angles exactly 30°, measure the angles you got and use them as reference for the other parts. For now it is only important that all parts are cut with the same angle.
Out of a thin (2-3mm) board, cut with the jigsaw some parts which fit into the windows. They have to be bigger than the holes but smaller than the groove of the rabbetting bit. You can use the wall boards as a template.
Out of the polystyrene glass you will need parts of the same geometry. Instead of cutting it with a saw, scratch it with a knife and break it. I used a pair of nippers to make the corners.
Test to see if they fit; correct them if necessary. As it is hard to make all windows identical, mark which part fits into which window.
Cut 3 ribs. Measure the angles and dimensions of the boards and transfer them. The "3cm" dimension in the photo must be adapted so that the screws used in the next step reach the outer walls but do not go through them.
Drill holes to place the screws for the next step. The holes' diameter must be larger than the screws used.
When the glue is fully cured, round the corners of the door with the sander. I used first a belt sander with 80-grit sandpaper and an orbital sander with 80-grit sandpaper. Be very careful -- if you are not used to belt sanders, use an orbital sander with 60-grit sandpaper.
Sand the whole front with an orbital sander and by hand. First use 80-grit sandpaper, then 120-grit.
This next step is complicated to explain. First glue a rib on top and bottom of the 80x60 board to enlarge the hinge area.
Then determine the correct angle to cut out the hinge hole by laying the door onto the 80x60 board and holding the hinge onto it. Note that the hinge has to be placed so that the door closes with a little bit of pressure remaining.
Cut the marked part out of the 80x60 board. As this cut has a difficult angle, I used a hand saw. Make the hole large enough so that you can reach the hinge screws from the back when the door is closed.
Now finally join the door and the back part with the hinge. As the wood is very thin near the hinge, drill holes before placing the screws. Test to make sure you can open the door.
Choose a decoration for the windows. I chose the wallpaper we already used in the room. But you can use anything flat, like dry flowers or funny pictures. Normally it is better to affix it onto the boards with tape.
Put the glass into the window.
Put the board into the window.
Drill a hole for some small screws to affix the board.