The first step is to cut out the underbase for the tree canopies. I like to use black foam core as its cheap, durable and the black hides any gaps from the attached foam material. Just trace the shape of each tree base onto the foam core and the cut it out with a new facto knife blade. For some added elevation I attached another layer of foam core. In actual practice this didn't really do anything so I would skip that step.
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Some of you might notice the lines of hot glue running form the edges into the center of the foam core. That was done on purpose to control the hot glue strings. One you're done putting a bead of glue along the edge, drag the tip of the hot glue gun along the top to get rid of any excess glue - that reduces the wisps by 95%.