For smaller projects, I like using this paint pouring medium by Artists Loft from Michaels. The ratio is 1 part medium to 5 parts paint so a little goes a long way for smaller projects.
Ignore the pens because originally I thought I was going to do these w/ an alcohol ink technique and darn if my Sharpie wasn't as dry as the desert in which I live! The ornaments are the point here. I picked up a couple of these from Michaels not knowing at the time what I was going to do with them and of course now I'm kicking myself for not buying more because I LOVE THEM. They take the paint really well and aren't explicitly Christmas.
Clean your ornaments with alcohol to remove any residue, fingerprints etc. Mix your paints with the medium per the instructions on the medium. Now you're ready for your pour!
I did what's called a clean pour which means pouring each color individually vs a "dirty pour" where you can layer the paints in a cup before pouring. Pandemic takeout has been good for my craft supplies since I re-purpose the containers for craft projects!
Start with any color, pour a little, then pour the additional colors in them middle as shown until your surface is covered. You want the paint to drip down the sides to continue the pattern. I placed the ornament on an upside down cup to give it drip space.