The Locrian mode starts on the seventh scale degree. It is the least used mode, in fact you’ll find very few examples of its use. In fact I can’t really remember ever using myself.
In the Key of C : C D E F G A B C the seventh scale degree is B so we have an B Locrian in this Key.
B Locrian: B C D E F G A B The intervals are therefore S T T S T T T
If we compare to B Major : B C# D# E F# G# A# B we can see that the Locrian mode has a b2, b3, b5, b6 and b7 compared to the corresponding major scale. The b5 is quite unusual; the locrian mode only really fits over diminished chords and doesn’t really have any chordal harmony. It has a tense, unresolved sound.
An example pattern starting on the A string is shown below. I’ve started on the A string to keep the fingering pattern fairly uniform. As with all the modes, you definitely want to experiment a lot with playing this pattern in as many different positions as possible (audio here):
If you need a diagram showing the notes on the fretboard, one is available in the Ionian mode (Major scale) post.
I’ve included a very simple example over a Bdim chord to get you started (audio here). By all means experiment, you never know you might find a sound you really like!