I make bunches of these every year. I have a tree full of them, tradition is when someone stops by, they get to pick an ornament off the tree. This year I really tried hard not to do so many ones that look like fire (my favorite) Really easy and great looking.
if you find already decorated ornaments, or scratched up ones at a yard sale or second hand store, here is how to turn them into clear ones.
Take your freshly clean and dry glass ornaments
Bottles of actylic paint.
Place three or four drops of a couple of different colors in the ornament.
Remember, the more paint the heavier and the longer the drying time.
Tilt the ornament on it's and let the paint follow the pull of gravity
You can find something the ornament can lay in on it's side and rotate it a bit every 15 minutes or so.
If you think it needs more paint, add a few drops but don't over do it.
Keep rotating the ornament.
I have one sitting and waiting to rotate while I work on another, so I end up with several going at one time.
Keep in mind that until the paint inside finishes drying the pattern will change.
Remember as they hang upside down it will change, so it is best not to over mix them.
I know it is hard and not shake the crap out of it to cover everywhere, but it is so worth the wait.