I then get a piece of muslin, cut it to the required size, and make a mock-up if the item. This dry-run helps me to firm up the measurements and make sure that everything that works perfectly in my mind works just as well in real life.
And then I make a real prototype. I almost always make it in fabric that coordinates with my tote bag, since I'll be carrying it for a while to make sure that the design will stand up to daily use. The prototype gives me a final chance to make sure that my new design works (muslin is great for a dry-run, but is a bit thinner than quilt-quality fabric), and that the end product will be the quality I need it to be to be comfortable selling it. It also gives me the opportunity to make something for myself - not a common occurance.
After a few weeks of use, I'm comfortable putting the new design into production and making items for sale. During those weeks of use, I often jot a few notes for adjustments and changes on the design sheet, promising that, if I can read what I've written, I'll be making an improved product.
I started my wallet journey with the Simply Chic wallet, just a simple two section envelope style wallet. Next, the Doubly Chic billfold-style wallet was introduced. The Trifecta recently followed. And now, the fourth, and probably final addition to the family is a clutch-style wallet, as yet un-named. This wallet has just finished going through it's paces. I really like the functionality of the new design and am ready to make some for the shop - it just needs a clever name!