As well as the guide and tutorial mentioned above there is a good on-line Grammar of Blazonry written by Bruce Miller of the Society for Creative Anachronism, most of which can be applied directly to Drawshield. If you would like to learn more about Blazonry and Heraldry there are many books available on the subject, almost all of which cover at least the basics of the blazon language.
Other Information on Blazonry and Heraldry