I know you can buy the tinting film yourself but if you try to DIY, you will fail.
Far better to let your local professional do it for you.
A fortwo is half a car so shop around and don't pay too much.
I paid £125 to have mine done through Shock Grafix in Dartford.
Look online, ask on forums, approach strangers in car parks and ask who did theirs.
When you find a company who is value for money and good at tinting, spread the word.
The plastic windows in the rears sides of the 450 fortwo and Roadster coupe are tricky to tint.
Plastic expands and contracts so the tinting film often bubbles.
Some tinting companies won't tint them, some won't guarantee the tint on those windows.
Some will sell you expensive "special" tint film for plastic windows. These bubble too.
There is nothing you can do to film tint these for any period of time.
Consider spraying them with tint spray or just standard spray paint.
Alternatively, try a product called Flyeye as shown here.