First you need to prime the rod, finials, and brackets with a good-quality spray primer. Apply a thin coat of primer. When it's dry, lightly sand the rod with 180 grit sandpaper until smooth, and then wipe off all dust with a damp cloth or tack cloth. Apply the first coat of paint with a foam brush and allow it to dry. Repeat with a second coat. Depending on the color and how well it covers, you might need to apply a third coat.
Here's a trick for painting the finials: Get a small block of scrap wood and drill two small holes in it. Then screw your finials in partway and start painting. This allows you to paint the finial without having to put it down and ruin your paint job.
Now it's time to get creative. To spiff up your dowel and finials, you can do all sorts of fun things, from painting stars and dots to glitzing it up with glitter! To paint dots, use a small, good-quality artist brush to make little circles and color them in. Or you can always resort to the pencil trick -- just dip the eraser into the paint and make a dot on the rod or finial. After making your dots and letting them dry, try adding a bit of glitter paint on top of the dots. The glitter paint is easy to use. Just use a small artist brush and apply wherever you want some sparkle.
Now that you've had fun decorating your dowel and finials, you need to put some polyurethane on everything to protect your nice paint job. Use a good-quality spray polyurethane to spray your finials while they are still in the little holder. Lean the rod against a wall (in a well-ventilated area). Spray in small even strokes and let dry. You can also use a regular liquid (water-based) polyurethane applied with a foam brush.
After you have finished all of these steps, you are ready to attach the finials to the dowel. Drill a small hole in the center of each end of the dowel and screw in the finials.
Note: You can usually buy a dowel, finials, and brackets at a hardware store or a lumber store.