This is one of the fun parts...choosing a paddle design. I won't go into the multiple types and forms of paddles, there's a fantastic chapter in Warren Graham's book Canoe Paddles: The Complete Guide to Making Your Own.
In addition, you can consider checking out Bark Canoes and Skin Boats of North America by Tappan Adney, a fanstastic ethnological look at the detail construction of nearly every native craft in North America. Many chapters include sketches of native paddles.
I like to use my meager Photoshop skills to scan any paddle images and manipulate the dimensions to customize the paddle. Shown on the left is a typical ottertail pattern blade that I screen captured.
Then, images are printed off (usually on legal paper), taped together to form the image of the blade, and the pattern cut out to be used as a tracing template. Pretty low tech. Warren's book talks about cutting out a 1/2 blade pattern from a thin piece of wood and then trace the image, but I couldn't be bothered with this step...so old fashioned paper and pencil prevailed.
Each image is carefully cut out to maintain symmetry, but a little error here is fine...most people don't have the accute perception to notice minor flaws anyway. I've also begun using photoshop to mark center lines in the image digitally, which helps align the pattern with the appropriate centering line on the lumber stock.
I've since learned that one need not bother drawing out the whole paddle (grip + shaft + blade) as the shaft and grip can be easily drawn in or customized on the lumber stock after the fact more accurately with a straigh-edged metal ruler. Instead, I prefer to choose the blade pattern first and then measure out dimensions afterword. More on this in the next post.