Get a used spray bottle, one that you can adjust the spray to be narrow or wide angled by screwing unscrewing the cap. Set it to wide angled. Fill the bottle with some water and cleaning solution. Something you wouldn’t mind being sprayed about. Surface cleaner is a good bet because you probably already have that around the house to clean with.
Creep up on a fly with this diluted concoction and let rip. It might try and fly away but the wide angled spray plus the slight stickiness of the solution will hamper it’s efforts. Watch as it falls to the floor and writhes about in agony as it desperately tries to breathe (flies breathe through their skin which is now clogged with cleaning solution). It’s wings will be useless now too as they’ll be weighed down by the solution.
Finally with some tissues mop up and squash the little shit. For the first several flies leave them where you let them die as a warning to other flies in the area. After a couple of days you can clean up the bodies.