Use a hot glue gun to adhere the moss to the inside of the metal lid. Make sure that the moss does not touch the sides of the lid so you can screw the cap back on the jar. Dab parts of the moss with black paint.
Cut the branches to fit in each of the jars and then paint them black, let dry.
Cut small bat shapes or silhouettes of witches or other spooky shapes from cardboard and paint black, let dry. Or find the shapes on the internet and print out.
Adhere a branch or two in the middle of the moss with hot glue. You may have to hold it in place for a minute or two. Then use thread to hang a spider — or spiders — off the branches, or you can glue spiders to the base of the branch. Add a bit of the cotton spider webbing to a couple of the twig branches.
In the next jar, make a ghost using the cotton spider webbing. Glue it to the moss and paint the eyes and a mouth. Add a branch and some webbing.
When your designs are complete, carefully place the jar over the top of the lid making sure not to move the pieces.