Watch the Modal Study #3 online guitar lesson by Jon Finn from Modal Rock Soloing
Jon explains and improvises using the C mixolydian scale. Note that the characteristic note (Bb, the b7th of the scale) isn’t used until approx. 00:31 seconds in to the clip. That's because the Bb is prominently featured in the chord background (Bb/C). But feature the note a lot in your phrasing to create that unmistakenly mixolydian sound.