To put up a Christmas tree without a stand, you will need to secure the tree in place using some creative methods. You can use heavy objects like large rocks, bricks, or even sandbags to weight down the base of the tree. You can also use a rope or twine to tie the tree to a wall or other stable object. If you have a large, sturdy box, you can use that as a makeshift stand by placing the tree in the box and tying it down with rope. Finally, you can use a combination of all of these methods to ensure that your tree remains standing upright and stable.