Watch the Modal Study #7 online guitar lesson by Jon Finn from Modal Rock Soloing

C Locrian is a Db major scale starting on its 7th degree. C Locrian is also a C major scale with b3 (Eb), b7 (Bb ) b6 (Ab), b2 (Db) and b5 (Gb). Essentially you start with a C major scale and flat everything except the 4th degree (F). It's a very unstable sound. As long as you can hear the sound for what it is, without expecting it to sound like a tonal center, your ears will get used to the sound. Use this over a Cm7b5 chord.