how should I practice to play riffs and sing at the same time? I'm fine playing chords and sing, but with riffs, I got lost. Is there any aproach? How did you sinchronize that?
It's a matter of slowing down your playing to the point were you can play it perfectly without thinking about it and then focus on the singing instead.

On tunes were i sing and play at the same time i've had to learn the guitar part (slowly), then work it up to were i can play it at the original tempo perfectly. Then go back to playing it slowly but focus on singing instead.
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Practice, practice, and more practice.

There isn't really a shortcut. Yes, sometimes this is hard, particularly if the rhythms are complex, but you have to get to the point where you can play the riff on autopilot.
And probably a bit more practice...
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I slow it down, match certain notes on the guitar with words that you sing. Skowly speed it up.
And no, Guitar Hero will not help. Even on expert. Really.