Before talking about the real deal, some basic thoughts in advance. Always have in mind that whatever theme will be your active theme: it's a fundamental part of your site. That said, twisting the look and feel of a live site can't be a good idea, right? Your users will gain a bad experience if menus are subject to change every 5 minutes or when they can't login any more because you chose a black font on a black background by mistake. Wrong coding may even lead to a 'white page' and your site will be offline completely. So, don't stress yourself and your community - always keep development aside from a running forum.
Making backups before applying any changes is another important section. Nobody is that tough to create a perfectly looking theme from scratch in just one step. At that means: It will take a lot of time and changes before you're done. Some changes may work as expected instantly, others don't. The real nasty ones use to appear to be wrong days or even weeks later. And it's sooo good to know that simply reverting back to a former backup will bring you to a still working stage. It needs discipline, yes, but the more backups you have the better the chance to localize where and when a bug became part of your code. All you need is a meaningful and consistant versioning system. You may either make it part of your file names like file-000.php to file-999.php, add a timestamp like file-20151004-2031.php - or even more professional: Move your stuff to *https://github.com/*.
The last prerequisite is
Never touch your files with document processing tools like MS-Word or OpenOfficeWriter. They add a lot of extra formatting tags and your code will become unusable. Use Notepad++, SublimeText or something similar instead. The capability of searching for a string in a bunch of files will become extremely helpful, since (at least in the beginning) your every day's development cycle will be:
Deriving a new theme from the OSSN default theme
As already said, it makes much more sense to apply your changes step by step to a known working template instead of starting completely from scratch. All you have to do is:
And have fun developing. :)