Doing a complete SEO Audit is not an easy task. You need to approach this from different angles, have a defined structure of what you wish to accomplish and use the right tools to get the job done efficiently.
In the past, only a few experts were able to tackle such a big task. Nowadays, there is so much information on this topic and there are so many free tools available that it has become very practical for any of us to perform complete SEO audits for the sites we own.
See on www.stateofsearch.com