Jumpstart your DIY Overalls Conversion by searching for the 80s music video jam-packed with overalls - "Come On Eileen" by Dexys Midnight Runners, then find - in your own closet - or buy, at a thrift store, overalls/dungarees/salopettes with an attractive fit for your torso and with straps that you like. I visualize the kind with a waistband similar to that of jeans... sometimes there are buttons at the sides. Cut off the pants part of the overalls - below the waistband... no need to worry about fraying if you cut neatly below the seam.
Also find or buy a skirt (that fits you) with elastic-waist or drawstring-waist... this will provide the skirt for your Salopettes Pinafore. Keep in mind - you will be removing the skirt waistband, which means the skirt will then be slightly shorter, so you'll need a skirt that fits a bit longer than you like. (Another option for this DIY Salopettes Pinafore Conversion, is to cut your skirt from a dress, or simply sew your own skirt with a fabric you love.)
Neatly cut off the skirt waistband. Cutting the skirt from a dress will work also - either way, the width of the skirt must either be equal to, or greater than, the width of the overalls-bodice.
After detaching the skirt's waistband, press the fabric.
To prevent fraying of the skirt's waist-edge - finish the edge by turning and stitching, trimming the edge with pinking shears, or using a liquid edge/seam-sealant. I pink or turn-and-stitch edges, but here's an interesting link in which blogger Shannon Sorensen tests the efficiency of edge/seam-sealant and offers photos for analysis.
Place the skirt's waist against the overall-bodice waist and if the skirt's waist is bigger than the bodice waist, you will now need to gather the waist with basting stitches - by hand or machine. Distribute the gathers in an attractive fashion - keeping them in place with more basting, pins, or masking tape.
You join the bodice and skirt with either hand-stitching or machine-stitching, but it may be easier to use hand-stitching and take small stitches into the inside of the overalls-bodice waistband.
At this point, you're on your own... if you need help joining the skirt and bodice, there are online tutorials - search "gathering a skirt and attaching a bodice".
With ingenuity, you can also attach an overalls bib/bodice to pants or shorts.
Launching your garment conversion with already-existing pieces results in the wonderful realization that half the work is finished!... The hems are there, the bodice straps are there... hip-hip hooray, merveilleux, meraviglioso, wunderbar!... happiness, bonheur, felicita, Gluck!
For more, see the original DIY Salopettes/Overalls post... the almost most popular post at Toile La La (from the time Toile La La branched into Art Fashion Creation).