Best way to make guitar and bass riffs?

So I sing, record bass, and not sure whether I wanna play rhythm sometimes or all the time. Anyways I have a few songs written and my guitarist created a bass and guitar riff for one of them and I have a bass riff written for another. Well I just wrote what will probably be a heavy song with some Slayer like lyrics and some Manson like vocals and Megadeth style 2 guitars. I'm having trouble with this because it's such a short song and I need to work on a solo and bass riffs and all that. I have an intro down for both bass and lead guitar but am having trouble coming up with some other riffs. So does anyone have any tips for me on how I can make it easier?

A riff is a set Rhythm pattern, a song only needs one riff, you just move it around with the chord changes. I assume you are into Nu Metal so one riff is all you need, think ‘Smoke on the Water’ by Deep Purple or Black Dog by Zep. I know its not Metal but you should really listen to ‘Crossroads’ by Cream, Jack Bruce is a Bass Riffing legend and this song is a good example, he's also the lead singer, leaving Eric Clapton to get on with his chops, if theres a video of the song on Youtube, all the better