Determine the length of moss you need for branches
Depending on whether you want to make a tree for a Christmas Village scene, a Christmas Railroad, or a Dollhouse, you will need a particular size of miniature tree. Insert a single stem piece of lycopodium moss into the centre of the bottle brush at the bottom of your tree to get a sense of how long your pieces of moss will need to be. You want the moss to protrude beyond the end of the bottle brush, but not so far that the tree becomes too wide at the base. Trees that are five to six feet wide in real life are normal indoor Christmas trees, try to create that diameter in scale for your miniature tree, or plan on something much larger to represent an outdoor tree.
When you think you have a suitable length, separate five or six other one or two branched segments of moss from a clump and trim them to a similar length.
Glue the branches into place
Dip the ends of the moss segments in glue and use tweezers to carefully insert them into the centre of the brush, beginning at the tree base. Work all around the tree, turning the tree as you go and inserting branches so they droop down slightly (the weight of ornaments will cause them to droop a bit more after the tree is made). Try to get the branches the right length by cutting the pieces of moss at the bottom end. You will be able to trim the tree to its final shape later, but the more complete ends of moss you leave, the more realistic and less pruned your tree will look.