As with everything, practice makes perfect.  I felt pretty much the exact same way when I was starting off.  Usually, though, if I worry too much about going back and fleshing things out, I lose the drive that I had to help me move forward.  So I just take notes, either on paper or on Notepad, depending on your medium, just giving a brief description of what I wanted to fill out and how, and just keep going.

Of course, every person's technique is different, and if you feel better going back and fleshing them out, and then continuing, then by all means continue that.

Matt