The best way is to either start with a blank textfile, then cut&paste stuff from other peoples themes while making notes about what the options actually do. The other way is to start modifying an existing theme and gradually changing everything, so that in the end only your own stuff remains.
You don't really have to worry about knowing how to do something, browse the themes, look at screenshots and try to figure out if some of them already has what you're trying to do. It should take aeons for you to even think of stuff you cannot learn from examples this way, and when you do, you're probably already mastered the art of learning itself.

And... Without experience there is little hope in getting stuff right the first time you try. So, for a base, get a random theme without VWM and without scripting, for they are much more failure-resistant. Build on that.