How can I paint flowers on old windows?

you could paint them or you could decoupage them (flowers from calendars, magazines, etc) here are two illustrations of cut images placed on glass. The glass is then painted. You could do this with a window too and either paint the background or use paper to cover the background. Each image (peach or pear or flower) is separately cut out. You get brighter colors using magazine prints than you may get painting on glass.

l would use a stencil purchased at a craft shop.

Dray them first with a sharpie then paint them you could even trace them like cut out a flower out of card board then trace on window

Get one of those adult coloring books with a flower design you like, tape it to the back of the window and on the front start painting! You could trace the black lines if you want or leave out for a more painting look. I’d go with crafting paints, look for good on glass.