Convert existing HTML5 site's CSS for mobile devices

I'd like to create a basic mobile version of an existing HTML5/CSS site. It must display nicely on iOS 5 Safari on an iPhone or iPod Touch, though I would expect a well-written mobile site to also work well on Android devices. The existing site has one front page layout and three different content page types, but they are very similar and just differ in arrangement of columns (single column, two columns, two columns with a double-width header) and all pages currently share a single CSS file. Some changes may be necessary to the HTML, but if possible these should be kept to a minimum to avoid disruption to the existing layout. I can provide an example mobile site that I developed previously as a starting point, though you don't have to use it.