Once you make it you right click and apply to mainhand. You sit for about a second. Then on the first or second attack it procs. One time use on classic i think they eventually made it like a short-term proc duration buff, stackable, etc.
Kinda pricey and time consuming but fun. It's situationally helpful too.
http://wiki.project1999.org/index.php/Skill_Make_Poison