Technically this stringless apron does have a string -- the neck loop.
I cut a strip 3" wide x 30" long. That was too long.
What was I thinking? 24" was better ... experiment.
Sew right sides together and turn inside out.
(While struggling with a safety pin to turn the tube, I kept thinking about my darling hubby who held up a tube turner and said, "Look! 85% off!" To which I replied, "What would I use that for? Ha!")
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Tuck the ends in to be tidy and top stitch.
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Attach to the edge of the bib on the front side ending about half way down the bib.

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Hand stitch on a cute button. I didn't have anything cute on hand, so these had to do. But have fun.
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Back to the neck loop. If you made it too long, like I did, figure out how long you want it to be and snip off the extra. Tuck one end into the other and top stitch.

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Extra ideas:
Decorate the bib with quilting using metallic thread.
Add some rick rack or fancy braiding to embellish.
Most importantly, have fun and don't sweat the measurements.