My sewing background is fairly extensive and I spent four years working in theater costume construction, so I may have some techniques that you're not familiar with even if you're a handy home sewer. Give them a try. I also have a small library of great reference books which I might mention throughout the instructable.
Step 1: Find Inspiration
First, they show you the pure style idea with no distractions and often in several variations.
Second, the illustrations tend to show seam lines and darts, which makes it easier for me to draft the pattern myself if I want to.
Third, and possibly most important, the back of the envelope has yardage indications for each size and for various widths of fabric. Very, very useful.
You can also look at finished garments, or dream up something in your own imagination.
I came up with this sketch after I'd seen the fabric I knew I wanted to do something with. I made a couple of changes during construction (I made an attached collar rather than two separate ones, and there's no bust dart), but otherwise the finished shirt is corresponds pretty much to the design and I was very excited with it.