How do I get rid of flies?

Open windows. They fly out. This is not a silly suggestion. It works.

Since I bought a fly screen for my back doors we haven't had a fly in the house they don't seem to come in the windows who knows why only the doors. I wouldn't use spray either you end up with dead flies everywhere. You can get the fly screens very cheaply on Amazon and they just stick on with velcro pads so you can quickly take them down again if necessary.
It also deters the neighbourhood cats from coming in the house when I'm not looking.

We lvie near farms and horses, so we got a fly screen fitted for one window per room- and it made a huge difference.

Put dishes of vinegar and also sodium bicarb in kitchen and other affected rooms.

But once they are in- I wouldn't spray in a kitchen or anywhere with food, children or pets- and the only safe and humane way is a fly swatter, and or those old-fashioned fly spirals with fly 'glue' - horrible but they do the job.

Spray!!