Or you could use this awesome extension by IWD Agency. Tons of great features, mobile friendly - everything you need in a one step checkout extension.
http://www.magecloud.net/marketplace/extension/one-page-checkout/There's another great solution by Sergiy Stotskiy, which I can't link to (only one link allowed). Just google "Sergiy Stotskiy 0 Step Checkout"
Or "Ulrich Abelmann One Step Checkout" - another great alternative.