User control to change master page from desktop to mobile version

I'm well stuck on this one :(

I have a public facing 2010 internet site with a custom master page/css - desktop version.

I've created a custom mobile master page/css for mobile devices - mobile version.

I need to create a user control that gives a site visitor the option of clicking on a link and changing the master page from the desktop version to the mobile version.

Where do I even start?