Doing SEO for another company's website

What is the best way to implement a full blown SEO plan if you wanted to become a third party SEO consultant?

Is it best to work inhouse on their machines to fine-tune, tweak, delegate tasks to their IT and design/copywriters?

Preferbly, I'm interested in downloading their site with dreamweaver and then saving a copy onto my own computer. From this offline copy I can fine tune and tweak the necessary areas of their site without putting their online version under construction or risk creating a glitch. Once the offline website I have on my computer is tweaked, I guess it can simply be given to them on a disk and they can have it uploaded and overwrite the previous web page files etc.

Is this the way SEO firms typically approach a project such as this? Do any other methods come to mind?

Obviously, somewhere along the line you would have to obtain to access their hosting account to view web metrics and whatnot. My question is more about what the best way is to get down and dirty with their coding etc., minimizing their site's downtime and margin for error or glitch.