So first I had to make my square piece of fabric with a slit cut to the center into a circle. To do this I folded the square in half, and in half again and then diagonally. So that the center of the fabric remained as point. I measured from the point to 20 inches out into the fabric and placed a pin. I did this along an arch and then cut along the pin line leaving me with a shape like this.
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Once opened this leaves me with a pretty darn nice circle
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With both the top and bottom pieces of fabric now in the shape and size I wanted I decided to join the pieces with bias tape. So I purchased a 1 inch bias tape maker thing, and followed the directions and made enough with my leftover base fabric to edge the circle and go up both sides of the slit. Here it is sewn onto the top layer of fabric.
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So the next step was that I cut out some cotton batting and basted it onto the base fabric. Just to hold the layers together.
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Then I pinned the top and bottom together, right sides facing and sewed the back onto the other side of the bias tape. I left the center hole open so I could turn skirt right side out when I was finished.
Here is the nearly finished skirt posing with our tree...
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What I learned
While I am quite happy with the end look of the skirt, there was things that could be done better. The top layer of fabric should have been cut about an inch smaller all the way around to compensate for the bias tape. Now the top fabric is larger than the bottom fabric. Not a big deal in my case because I liked the lumpy look of the top fabric, and it is light weight and drapes nicely. I could have found a better way to deal with the transition of the bias tape from the circle edge to the slit. I just kinda fudged it and made it bend. It is behind the tree though and I am going to live with it. I am thinking of top stitching along the bias tape edge to kind of hold it down or something. Still have to hand sew some of it together... but I think it is done for this Christmas. Afterall, the tree is wearing it and looking quite dashing. It can be finished another day.