I don't know of any links or apps to help with pattern writing, but I started pattern writing last year. My suggestion would be to take a pattern that you enjoyed making with good instructions. See what it entails and make sure you have everything important in your own pattern. I would also suggest getting someone to test knit it for you and give you feedback.
Important things to list are: All materials needed to complete the item including weights and thickness of yarns, needle or hook sizes, size of buttons used etc, stitches used, sizes available in the pattern, page numbers, copyright info, can the buyer sell the completed object, clear instructions on sewing the pieces together, and photos. Hope this helps!