Use Q Tips
Because window painting can be a rather delicate procedure it is always useful to have Q-Tips. They can easily reach what regular brushes can’t, and they are good for putting a finishing touch on tiny cracks and crevices without affecting the wall paint. Try to get the pointy ones as they allow for much more precision.
Use consistent colours
Make sure you have enough paint to do a given job. Paint colour can actually vary between cans (even if they are actually the same) so you should make sure that you use the same can for the project you are working on.
Push as you paint
Use a pushing motion while painting to prevent the paint from running down and looking like drips. Drag the paint slowly and let the bristles cover the area thoroughly.
Avoid using tape
It is best not to use tape to avoid getting paint on the glass of the window. Instead, just scrape the paint off as mentioned earlier. In addition to this, make sure to leave the paint that lies between the wood and the glass, otherwise the wood can be exposed to moisture and rot.
Place a rubber band around the can
Place a rubber band around the can so you can use it to wipe off excess paint rather than the rim of the can. That way you can easily put the lid back when you are done.