As soon as you have a new job requirement. The light turns green and the race to find a candidate begins. Below are some steps that I would do as soon as a new req opens up.
1. Find out the basic information of the position (e.g. salary range, job title, job description, qualifications, location, etc)
2. Post the job as quickly as you can with as much information as you have. Post it on your website and post it on any Career Boards (Craigslist, Monster, CareerBuilder, etc)
3. If you have a "hot list" of candidates call them first. A "hot list" is a list of candidate that you've spoken with in the past that you know are qualified for this position. If you don't; have a "hot list", now may be a good time to start making one.