1) Make the same website, but as a 100% width website.
2) Shrink your browser to 300 pixels wide to see it as people will see it on their phones. Expand and contract to make sure floats work properly.
3) Tell the browser to use native width:
<meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0;" />
Here are some examples
http://www.ballroombook.com/search?usemobile=0
http://www.ballroombook.com/search?usemobile=1
http://www.johnscraigslisttool.com/?usemobile=0
http://www.johnscraigslisttool.com/?usemobile=1