The first step is to know what you want to make! I decided to only use the glass we had on hand rather than go out and buy anything new. This limited both the size and subject matter of my design. Since we had some lovely red-orange streaky bits of glass, I decided to draw up my favorite flower - the poppy!
I did a google image search for poppies and did some thumbnail sketches of some photographs I saw. I decided on this image from http://poppieshealingnetwork.wordpress.com/ for my final design. I had to simplify some of the edges to make it workable in glass.
I then took the thumbnail I had made and blew it up to the actual size of the finished piece. Again, this size was determined by the amount of glass I had on hand to work with.
Graph paper makes this thumbnail-to-working-design process a breeze. Every square in the thumbnail became three squares in the final design. Easy!