You'll need a large hollow, hard plastic ornament. They are fairly common; these came from Home Depot.
Cut two or three holes in the top of the ornament for filling with potting mix and for placing your plant into the mix.
Draw the holes with a pencil or pen, allowing enough ornament to remain between the holes to support the heavy planted ornament; 1/2 - 3/4 of an inch should be fine.
Cut out the holes with a saw; there's enough curvature to get a slice started and enough room that you can work the blade to form a corner. This is good for a straight sided opening,
or use a hole saw, good for round holes.
You could also use a utility knife blade, or a razor blade held with a pair of vice grips. Heat the blade on the stove top till it gets really hot, about 10 seconds will do, then mark the ornament gently until the blade breaks through, reheating when it cools (it takes about 10 seconds to cool also). For your safety, you don't need to press very hard. Reheat often and let the heat of the blade do the cutting for you. It will also re-fuse as it cuts, so after working your way around the hole, you have to reheat then go back and free your cutout from the ornament. This method will cut a round, straight or wavy sided opening.
Be sure to put a few holes in the bottom for drainage, then fill with potting mix and plant.
This is an ornament planter from last Christmas and you can see nothing of the hole, so don't worry too much about the shape of the hole itself.
It's best to replace the tiny thread that was used to hang the ornament with something stronger; here I've use a plastic wire tie but a bit of wire or heavier string will be fine.
MERRY CHRISTMAS TO ALL and to all a good night.