Getting started is sometimes the hardest part.
For testing mobile design, you can simply resize your browser to see how the media queries are responding. literally drag the window and make it more narrow – height doesn’t matter.
I have an AT Core subtheme that I have customized. Using a subtheme is important so the core can be updated without wrecking your customizations. Make your first change in the file –
responsive.smartphone.portrait.css – if you have subthemed correctly, this will be in the css folder of your subtheme.
in my case, I changed the font size to something goofy – 212% using the wonderful apatana studio. I save changes, resize the browser window and watch the font expand to the new size. It doesn’t look good; that isn’t the point. All I have shown is where specifically to make the change and test the change you have made.
Mobile settings with narrow browser. Note the now ginormous font!