Right now, I'm in a band and I'm the guitarist as well as a singer.
So far, I haven't had to play a song where i have a guitar riff to play while singing.
I'm currently trying to play Reptilia by The Strokes and I can't play the riff without making a mistake if i'm singing and vice-versa.
What should I practice to get this right?
In practice, imagine the worrds in your mind as you play through the riffs. Do this till you can start adding the words, maybe just say them, then sing them, also try humming the words. It's like riding a bike, or walking chewing gum…takes practice. Study Dylan.