What is the best way to go about converting an existing website over to WordPress?

I am trying to convert and existing web page over to WordPress for a nonProfit organization that I work with, so that the Executive Director is able to update the site herself.

The web address is: http://www.vine-mn.org/

Really the only thing that needs to be ported over in the convertion is the visual look / feel of the site. Is the best way to do this by creating a template from scratch, or trying to move the image files & CSS into an existing template, or something else altogether?