How do I paint an old glass window to use as decoration outside?

Drawings are beautiful but it you aren't an artist you can always use stencils

Print out something you like and lay it under the glass as your guide. If you want it to look like stained glass there is liquid solder for the outlines and stained glass paint. If you use regular paint you can do the same and if you plan to put it outside you need to cover it with a resin or polyurethane or the weather will take your paint off.

I would go with the usual formula, Scd, adding your own touches as you go. Clean the window on both sides, wiping off as much old paint as you wat. Sand in any areas where there are splinters, sanding or leaving any paint chips you want. Stain or paint the wood, if you wish. If the window is almost all white, you can use a small paintbrush or cotton balls dipped in color and just daub color where you wish.

For the panes you can paint them with almost any kind of paint, from actual glass paint, to diluted house paint. You could mix craft paints with white glue to get variegated shades from a single brush of paint. If you want the painting to look like stained glas, just outline each “piece” of different colors with a black permanent marker. When you are finished, and the paint is completely dry, spray spray everything with a clear waterproof sealer. Leaving the window outdoors will cause it to break down faster, but it might look even better. Under a porch or patio cover, the window may last longer, but it won’t be the star of your garden. Best wishes as you work. ☺️

Here is how I used one in my garden: https://thehoneycombhome.com/diy-raised-herb-garden/