WHAT YOU NEED:

glass jar with tight fitting lid
distilled water, or boil some water and use when cool
glycerin (helps to slow the falling snow; available in most drug or craft stores)
white or silver glitter, of fake snow crystals 
clear silicone sealant (a tube of aquarium sealant is great)
small waterproof ornament or figurine
plastic jar lid, to fit inside the jar as a base to elevate the ornament within the jar 
another plastic jar lid, or wooden circle, larger than your jar lid to use as a stand
acrylic paint for the ornament stand, in your choice of color
coiled Christmas rope or decorative ribbon
craft glue to attach the decorative rope

WHAT TO DO:

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Check to make sure that the ornament you have chosen will fit within the diameter of your jar.
You will probably find that the ornament needs to be elevated somewhat to be properly visible within the globe. Use a small plastic bottle lid to create a base; the white plastic lid that I used to do this is visible in the photo, showing as a white area just below the ornament.
Make a trial run. Invert your globe jar and remove the glass part, leaving only the lid. Place the plastic base in the center of the lid, then position the ornament on top of this. Place the jar over the ornament and check the fit. If the ornament is high enough to view clearly, use silicone or aquarium sealant to glue the ornament firmly to its plastic base, then glue the base lid to the center of the inverted globe lid. Allow this assembly to dry and set, using the cure time recommended by the sealant manufacturer.
Use another small jar to test the snow fall. Fill the test jar with distilled water, or boiled water that has cooled, then add a scant teaspoon of glitter. Put the lid on and shake it to see if you like the effect. If the snow is too sparse, add a little more glitter. Add a few drops of glycerin and test it again. The glycerin should slow the rate of the snow fall quite nicely.
I have read that you can use mineral oil instead of water and glycerin, but I haven't yet tried this method.
Working over a sink, pour the water and glitter into the globe jar, filling it to the very top. Invert the ornament assembly into the water and screw the lid into place. Some water will be displaced and spill over the top of the jar. Wipe the jar dry and shake it up! I enjoyed this part the most, I think, because the little church looked so pretty in the midst of a glittery snow storm.
You can use the sealant to glue the jar to a wooden base or another plastic lid, if you like, for extra stability. Although I did use a wooden base for my snow globe, I chose not to glue the two together - this way, I can just pick up the glass jar to shake it.
Paint the base and the jar lid in your choice of color. I chose a terra cotta brown which looks quite effective.
Once the paint is dry, use craft glue to add your choice of Christmas coiled rope, a circle of Christmas lace, or whatever you wish to add as embellishments.
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