First, pick the fabric you would like for your skirt (about 1 1/2 yrds), then cut it into a big circle. Cut a small circle in the middle of the circle. Then cut up one side of the large circle so you can actually wrap it around your tree. You can put a velro clasp or button here. Now for the RUFFLE- Cut fabric strips about 8" wide, I ripped mine because my fabric allowed it and it created more of a raggy edge.
You will need about 7 or 8 of these strips depending on how ruffled you make it, and how large your tree skirt is. Now, fold in half (hot dog style) so you now how about a 4" piece.
Gather and fold, gather and fold, gather and fold..........
You can make the edge as ruffled as you want by how big of pieces you gather and fold. You can make it as different as you want by which way you fold your fabric (away from you, towards you). Have fun and be creative. Keep sewing your strips on until you get all the way around your circle.
Also, if you are in the same situation I am, with three trees, you can set that small circle piece (cut out of the middle) right in the middle of it, to fill in the hole and make it look like a skirt made for three trees. :)