|Top-stitching or felling is much more difficult once the skirt is assembled, but must be done AFTER the fit is correct, as it's such a pig to unpick. To make it easy, assemble the skirt like this:
Sew (but don't top-stitch or fell) all seams but one - perhaps a side seam without the zip.
Firmly baste the last seam.
Check the fit and make any adjustments.
Unpick the basted seam and do the top-stitching or felling.
Sew and top-stitch or fell the last seam.
This allows you to do almost all of the top-stitching on a fairly flat garment.