Use some tall glue bottles or coffee cans (whatever you can find that's tall and heavy) to prop up your tree while the epoxy resin dries. After the resin is dry, paint and flock your base and your tree is ready to decorate your battlefield! (Or Christmas village, train layout or diorama.)
I also use greenstuff and form it around the base of the tree, then I sculpt some rudimentary roots. This adds a little extra detail but also helps stabilize your tree. From here, you can prune and shape your tree as you desire. The more branches you have in your tree, the more you will have to work with in shaping your tree.