Character sheets are built using HTML, CSS, and Javascript. You can find more information in the wiki . If you haven't coded before, I'd recommend taking some HTML, CSS, and JS courses via something like codeacademy or w3schools (I believe both have free options). If all you're looking to do is add or change skills, I'd recommend using the tools/custom skills area of the existing 5e sheet as the work to accomplish these small changes will be a lot.