Start by laying the overalls out flat and cutting the pants off about 1" below the bib. Serge or zig-zag the edge.
I cut the fabric for the skirt about 17" long allowing for 1/2" seam allowance on each end and 30" wide. I gathered the top with my ruffling foot, although you could easily gather it with a long basting stitch. Then I sewed the gathered edge to the bibs using my zipper foot so I could get really close to the bib's seam.
I then added a ruffle of a similar fabric to the bottom.
The bibs then needed a little decoration so I took the pocket off and added a ruffle. I sewed the pocket back on using a gold thread that matched the topstitching that was already there.