Then you learn the hard way, by fixing them <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
Seriously, what we did besides getting a head start from Marcus was studying literature like "High performance sailing" which have some specific data on what a high performance sail should look like. Pictures of the top guys also helps when starting out, but you soon notice that very few pictures are actually helpful when designing your sail. Loads of fun, and all part of the learning process <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Be advised though, it takes time and work!