As requested, here is one of the patterns I use for tree skirts. You can use as many or few fabrics as you want; you will just need to adjust the size you cut your strips.
First of all, you will need the correct ruler. I use Marilyn Doheny's 9 degree circle wedge ruler. In order to get a circle, you will need 40 wedges. I've also seen a 10 degree ruler at my LQS. If you use that ruler, you would need 36 wedges for your circle.
You will need a total of about two yards of assorted fabrics for the top, 2 yards of fabric for the back, 2/3 yard of fabric for the binding, and a 54 square of any kind of batting. Please note that all seams are 1/4. This makes a 50 tree skirt.
For this skirt, I'm using seven fabrics. To the left of the fabric is the size of each. Each of these is cut by the width of the fabric, then they are sewn together lengthwise. When you are done, you will have one panel that should measure approximately 20-22 inches by 42 inches. You will need three panels for the tree skirt top. Remember to adjust the size of each if you increase or decrease the number of fabrics. Also be aware that each of panels will be cut in both directions, so you'll want to consider placement of each fabric in both directions before you cut your strips.
Sew strips together, and press all seams in the same direction. Once you have your strips sewn together and pressed, begin cutting out your wedges, alternating the ruler so you are cutting in both directions. You should be able to cut 13 to 14 wedges from each panel.
Here is the result of the alternating wedges.
Quilt as desired; I stitch in the ditch vertically between the wedges. Then bind as usual. Here is the finished tree skirt.