NOTE: you will need a few days to work on this because the spray foam takes a long time to dry properly, and you need to spray it section by section.
OTHER NOTE: try to do this outside so when you have to spray paint it's not a huge hassle to bring it in and out of your workspace.
Step 1: Supplies
Here's what you need:
NOTE: the amount you need of each material will depend on how tall or wide you want the tree. I'll tell you exactly what I used and how I built it, but you may need to adjust for your needs.
-2 big pieces of cardboard tubing, one being slightly thinner then the other so it can slide nicely in and out the other (one pictured above)
-at least 6 cans of spray expanding foam (also pictured above)
-dark and light brown matte spray paint
-fake moss (brown and/or green)
-trash bags or a large roll of paper
-hot glue gun
Step 2: Set Up the Tubes
Step 3: Spray Foam
So with it still laying on its side, start spraying strips down the entire side (except for where the roots will go). It will look weird at first, but give it a few minutes, and you'll see how the foam expands and starts to look like knots in a tree.
You want as much texture as possible here, so as you're spraying, if you have small pieces that are leftover hanging on the tube from the can, wipe it right off on top of another strip to make it look like an extra knot. Once you start working, you'll see what I mean. You'll get into a rhythm and start building your own growth patterns and textures.
Do this section by section, because if you don't give the foam enough time and you role it over, it's gonna get squished. You may need a few hours between each section. If you're pressed for time, you can do what I did and balance one end on a chair so you have it at and angle and can keep spinning it without it hitting the ground.
Step 4: Roots
When your shape is figured out, take your trash bags or paper and cover all the chicken wire, like its and actual tent.
Take your spray foam and continue the patterns you've already created down into the roots so you can't see the seams between the wire and the tubes.
Step 5: Paint
Once that's done, get the light brown paint and take a few steps back. Lightly spray it towards the tree. You don't want solid spots, you just want to use this as shading and to mess with the color. Up close it should look like you're just speckling it.
Step 6: Moss and Ivy
Take your moss, break it up into pieces, and hot glue it all around the tree. Just make sure it looks natural, which isn't too hard to do with moss.
The fake Ivy usually comes in strips. If this is the case, glue the top of it up high on the trunk and wrap it down as you go.
Step 7: Set It Up
If you want branches for the top, sculpt them out of chicken wire, cover up most of it in paper, and make the same pattern with the foam. Then use the wire on the end to hang them off the top of the tube.