Octave notes on the guitar can be played with the following string combinations:
Here is the pattern for the C scale, using the 5th and 3rd strings.
C Scale Octaves: 5th-3rd String Pattern
The use of harmony notes is only touched on briefly here. For a more in-depth study of how to use 3rds, 6ths and octaves, see Progressive Funk and R&B Guitar Technique, Progressive Blues Guitar Licks and Progressive Blues Guitar Solos - both by Peter Gelling.
Octaves can be played by using the pick for the lower note, and the third finger for the higher note.
Octaves can also be played with the pick, by "deadening" the middle string between the two fretted notes with the first finger. Thus all 3 strings are strums without the middle string being sounded.