With current heatwave, I looked for a way to insulate windows ; I tried taping space blankets, but any breeze makes them noisily flap like crazy. Then Inspiration struck me : I remembered how they have wraps stick on cars, I tried, and it worked !

Supplies

Space blanket(s)
water sprayer
credit card or similar
really sharp kife

Step 1: Prepare

Get supplies : you might already have most of these ; space blankets are cheap, I got mine in a local sporting goods outlet for 3 € each. I want to stress the need for a really ( I mean really ) sharp knife to neatly cut the space blanket, the thing is really thin and tears off easily, so a razor sharp blade is needed. I used a ceramic kitchen knife.

Clean your window panes' outside face. You don't want to reflect heat back to the glass, it would diffuse heat inside and risk cracking it.

Step 2: Apply

Don't try in direct sunlight, or you might suffer terrible heat as space blankets are very efficient at reflecting heat. Take space blanket out of its package, unfold it with caution, it's so lightweight it might make its dream of being a kite come true in no time.

Spray water on window pane and apply space blanket, with the silver-colored side facing outdoors : it will adhere 'magically' on glass. Smooth out bubbles and wrinkles with the credit card (I used a loyalty card) like you would for wallpaper.

Step 3: Trim

Now your window is covered with space blanket, carefully trim excess material withe the really sharp knife ; you may have enough material with a single blanket to cover several windows.

Step 4: Enjoy

Now your widows are insulated, stay cool indoors. Sometimes, corners loose adherence, so you need to spray a little more water. When I do that, corners get back in place by themselves. I suspect that very tiny goblins like to hold it close to the glass, but I'm aware it's not a scientific explanation of the phenomenon ; that's likely an electrostatic force of sorts, but I don't know which. Anyway, it works.

Step 5: Additional Thoughts

It does dim incoming light quite a lot, but still allows you to see what's going on outside. It also doubles as a privacy screen by day, at night it might be a bit too revealing ! I also like that it dims outdoor lights at night : I live downtown, there are so many streetlights and stuff.

It also reflects a fair amount of heat, so don't stand near it in direct sunlight. My west-facing window sill herbs do survive, but they're adapted to heat, regularly watered and are only sunlit a few hours. Additionnally, keep in mind it might blind passer-bys and neighbourgs. I added an awning to my east-facing balcony avoid this.

Reversing the process, i.e. having space blanket adhere on windows' inside faces with gold-colored side facing indoors will reflect heat inside in winter, saving on your heating expenses.