Buy glass etching stencils from your local craft store. They come in a variety of sizes and designs. Choose ones that fit on the glass balls you will be decorating.
Clean the glass balls first. Wash them with warm water and pat them dry with a paper towel. Attach a stencil design to a glass ball. Use painter's tape or masking tape to attach it.
Transfer the stencil to the ball. Most etching stencil kits will come with a wooden stick. If yours does not, you can use a craft stick or Popsicle stick. Rub the stick over the stencil to transfer it onto the glass ball. Do not rub too hard or you may rip the stencil. Rub in an up and down direction and then work left to right. You will know the stencil is transferred when it looks different.
Remove the painter's tape and carefully remove the stencil sheet. The stencil is made up of a blue sheet that you want to leave on the glass ball and a white sheet that you want to discard. Pull it back slowly and make sure it is fully transferred as you go. If you come across an area that is not transferred, replace the stencil and rub that spot the way you did for the original transfer. Once it is completely transferred, fully remove and discard the stencil sheet, leaving the blue sheet behind.
Secure the blue stencil by adhering it with more painter's tape. Overlap the tape on the stencil and at the corners. This will help keep the etching cream on the stencil and not on errant locations on the ornament.
Put on your protective gear. Wear rubber cleaning gloves, an apron that you won't be concerned about if it becomes damaged and protective eyewear. If you don't have a pair of safety goggles or glasses, you can purchase some at sporting good stores or online. You can also wear old sunglasses. Your work space should be well ventilated so that the fumes from the cream do not make you sick. Also, choose a space that is not far from running water. If you should get etching cream on your skin, you need to wash the area immediately since you are working with an acid that can burn you. You will also need the running water to rinse your glass ball.
Shake the container to mix your etching cream. Thoroughly coat the area to be etched with a thick layer of cream, using a paintbrush to apply it.
Set a timer for one minute. Place your ball, stencil still attached, under warm running water after your one minute is up. Leave it under the water stream until all the etching cream has been removed. Take off the stencil and painter's tape.
Clean your glass ornament with a spray window cleaner and paper towels, and allow it to dry before hanging it on your Christmas tree.