I usually write some guitar parts and have my singer come up with lyrics for it...

However he has a song written already... how do I come up with a catchy riff to play under it?

I kinda found some chords I can strum along with it, but I will be damned if I can figure out how to write a riff.. This is supposed to be a rocking song... not one with chords strummed along.

The chords I figured to play with it are C, G, C, G, C, G, D.... sounds like those would fit...

Its kind of got a feel like "Disease" by Matchbox 20

IF anyone really wanted to hear it.. I could get the singer to record a verse or something into a crappy computer mic....
Well writing rhythmlines to fixed solos is something i really suck at but:

Why don't you try to give the song a new structure for e.g. a different phrasing for your singer so that it'll be easier for you to play along (maybe you'll have time for fill ins)
I'm guessing you're mostly playing in G major? I would make a riff based on the relative minor (E minor in the case of G major). Em pentatonic is always rocking!
Don't try to make guitar to a song...that ends badly a lot. I'd suggest coming up with a bunch of songs and just seeing if it fits. A lot of the time I will already have lyrics written and then find that they fit some progression or riff I wrote. It's generally a bad idea to base one off the other.
Just wondering, did anyone write a singing melody?
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indeed demon. I was wondering how to put music under an already written lyric btu often enough I already make a singing melody.

If one could transribe that melody in Guitar Pro or other similar software, maybe you could loop it and try to jam over it.

You already seem to be messing around in G major as said earlier, so you have a place to start from. So it could work
I have written both ways. My singer came up with some good lyrics one day (anglo-celtic prog) and i just kept repeating the lyrics over and over until they formed sounds in my head. That ended up with possibly the best bit of music I ever wrote, so there's no fixed way to write and trying something different will often bring out the best in a song.
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