Place your half-foam circle into the box
You can glue it to add some stability
Leave a few inches of the dowel unwrapped so you can easily push it into the foam
Start with a dowel as your base, using long strips of naturally wired - about 15 inches long - and start wrapping the naturally wired around the dowel, going up in a spiral fashion. Leave gaps in the dowel is OK. You'll fill those in with the next few wires. Make sure you don't make it so tall that it's top-heavy.
If you want a thicker trunk, you can twist another wire around the tree. Make this one about 6-inches longer than the last one so you have branches at the top
At this point, I would push the unwrapped portion of the dowel into the foam
Now, starting about halfway up the tree, you can twister shorter wires around the tree to make more branches
Place those outward, and upward to look like tree branches.
You can repeat until you are happy with the thickness of your trunk, and amount of base branches.
If you want to use any leftover wire, add more secondary branches.
If you want to add flower blossoms, Just cut off a piece of the flower and stem, and twist around it to secure it in the branch. These are great for hiding imperfections
Add filler to your box and Cover with moss