I don't think there is a formula written in stone about acheiving this type of composing.Start out with your main riff as the foundation.Decide what key it's in and all the relative notes and chords that belong in this key.Experimnt with different combinations until you get the ideal blend you are looking for(sounds like mixing your favorite drink).AC/DC's highway to hell and Slayers Mandatory Suicide are a couple more examples.If you really want to get nuts, go outside of the key but sparingly(Primus comes to mind) or it can get nasty.