1 Make sure it’s a recalcitrant sash window. This is important.
2 For this to work, it has to be the lower half that needs closing.
3 Place your fingers over the edge of the window, and push down. You may need to push hard.
4 If you do this properly, your fingers will now be firmly jammed between the two edges of window. This will hurt. A lot.
5 Phone* the neighbours, who need to be out for this to work.
6 Phone the fire brigade.
7 Wait. This will also hurt. Try not to cry (I’m afraid I cried).
8 After a while, two burly firemen will arrive. With a fire engine. Although they don’t need the fire engine.
9 They will release the trapped fingers.
10 Voila! The window is now closed.
*you will need an accomplice for this, unless you have a phone on you.