Measure the height of the tree against the height of your ceiling to see if a taller urn offers better balance since, as a rule, an artificial tree looks best when it's around 2 feet shorter than the ceiling.
Choose a vessel with at least a 12-inch circumference if your tree is 6 feet tall; go wider for a 7-foot tall tree.
Follow package directions to prepare a mix of plaster of Paris (or any type of quick-set plaster) with water in your mixing bucket. Add the water slowly while mixing the plaster. Stop when the mix reaches the consistency of pancake batter.
Locate a plastic container that's small enough to fit into your decorative urn but not so big it leaves a large gap of space between the container and the urn. Pour the plaster mix into the container and allow it to begin hardening before you sink the tree trunk into the container's center. Brace or hold the trunk in place until the base hardens.
Lower the plastic container holding the artificial tree into your decorative urn. Add rocks, soil, newspaper, Styrofoam or another filler to fortify the tree and top off the project with moss or decorative greenery to hide the plaster and fill material. Fluff out the leaves and bend branches slowly to give your tree a natural look.