When I designed the medicine cabinet for our guest bathroom, I did something I've only done once before, and that's to make a full-scale drawing first. Usually, I just draw thumbnail sketches and a few tighter sketches of the trickier parts.
Making a full-scale drawing allowed me to work out the joinery details and the hardware size and placement. I was also able to step back to make sure the proportions looked nice. Normally, I just pull out my tape measure, measure the air and say "yeah, that's about right" when I'm determing outside measurements and hope they look good once the project is built.
Instead of actually drawing a design, a lot of woodworkers use Sketchup. I was an illustration major and am more comfortable with a pencil in my hand. Frankly, I prefer the warmth and personality of a pencil sketch over a computer printout; it's one of the reasons I prefer hand tools over power tools. That being said, I can certainly see the benefits of using a software program to design a piece. For one, you can rotate the computer image and view all sides.
I used Wonder Brads to install the molding. No need to predrill, they won't split the wood and are virtually invisible. I did not dab them with wood filler and no one has ever noticed them.