I would just grab a sheet that looks close to what you want (either close to the rules, or close to the look/layout, or both if you're very lucky), copy it into a game, and start tweaking it. Don't rely on the Preview window in the Roll20 editor; it does not always work. I open up the HTML/CSS editor page in one browser window, and the campaign itself in another browser window. Edit and save in the editor, then reload the campaign in the other window to check out how it works. I make frequent backups of my HTML and CSS as I go, saving multiple versions that I can go back to in case I screw something up (and I have needed them). Even once you find a sheet to start from, continue to check out other sheets. I have a number of games that are setup just so I can play with a particular sheet. If you find something another sheet does that you like, you can figure it out and then transplant/adapt it into your sheet. Unless you know your target audience will only use one browser, test in both Firefox and Chrome. Some HTML and CSS appear differently in each, enough so that it may impact the look and/or function of your sheet. Lastly, as TheAaron says, ask questions here on the Forums. I have had many of my stumbling blocks resolved by kind and generous folks here.
