I found an idea on Pinterest (originally from Lucy Designs) that I wanted to try out. But, since she doesn't have a tutorial, I thought I'd explain a few things about it that I was wondering as I went into this project.
What you need:
2 tomato cages (these were 56 inches from tip to end)
Garland (for my size of cage it took 6 of the 9 foot garlands, or 54 feet, for each tree)
Lights (I used two 100 light (24 foot) strings per tree)
The total height of my finished trees is 67 inches. As I was buying the tomato cages (yup, they do still have some at Home Depot) I checked the fit of the cages to the pots and it was perfect to sit in there as they were. They are not in any way secured to the pots and can lift right out. I love this because it means I can store the trees and use the pots throughout the year and when the plants die in the fall I can just set the trees over the top of the pot and into the rim.
Since I just bought these pots, the inside is still completely empty. If you live in a windier area I would suggest filling it with dirt or rocks. You could also stick a piece of pipe into the filler to tie the cage to so it wouldn't blow away. Luckily my entry way is protected so I didn't have to do this, despite the amount of wind we get here.
At the top I used a small piece of wire (twist ties would probably work as well) to secure the ends together.
Two people have come over so far and neither noticed it wasn't an actual tree till I told them, though it helps that the wreath is real so the front porch smells like trees. :) I may still put some ribbon on them, not sure, but for now I think they look really pretty.
I also really like the nighttime look of the cage and lights without garland, though I'm not sure how pretty it would be in the daytime. (See it here)