I'm a screen painter and I have instructions on my website on how to paint a window screen that may be helpful. Use thicker acrylic paint and a dry stiff brush, like a stencil brush. Base coat with a pale color or with white. Dab or spread the paint on gently. Let dry between coats. Place your screen on a black trash bag to help you see what you are painting. See my website for more tips. I'd like to see pictures of the Christmas ornaments.

Here is a link that might be useful: Anna's Painted Screens