We've posted an article with search techniques for the WWI Draft card database at Ancestry.com on our site:
http://www.rootdig.com/adn/using_world_war_i.html
You will need access to Ancestry.com to use the index (your library may have access onsite--ask them and find out, your local Family History Library has access). My suggestions for what to do when requesting a free trial are posted on our site.
Michael