We've seen a lot of designers & developers argue what the best way to approach responsive workflows is, but I don't think there's a universally right way.
We approach it by designing the PSD on a 12 column grid layout for a 1920x1080 monitor. It's then developed mobile first, using the designer's input as creative direction. From there, development is continued iteratively until a mostly fluid layout is achieved that matches the PSD.
Other people advocate designing entirely in the browser, designing only moodboards and elements, or going extreme and designing multiple layouts for every targeted device & width.
The only universal truth to all responsive design workflows is that developers have more artistic input to the final product than in traditional waterfall processes.