Q ~ Michael:  I am a bit fascinated with how a composer goes about notating interesting techniques such as knocks, taps and trills.

A ~ Randy:  Notating trills can be done with a "tr" or a specific squiggly line above note.  If clarification is need on which note to trill to, above or below the primary note, I would write a very small notehead with its pitch next to the primary.  The knocks and taps can have various names such as "tambora," golpe" etc.  For notating tambora, (knocks) I like to use an "X" in place of the noteheads.  I also back it up with tamb. for clarification.  Those can be executed on the back of the bridge allowing the strings to sound or anywhere on the body of the guitar.  I would also indicate the place of the tambora for a specific sound .  For golpe, I use a square box above the chord and write golpe next to it.  Every time it is repeated I only indicate with the box symbol.  Golpes are very specifically executed.  YouTube is a good source for demonstrations.  For nail taps, I replace noteheads with an "X" and write (nail tap upper side).  If executing percussive techniques on the guitar, it is imperative to not damage the instrument.  Using "tap plates" around the sound hole is highly recommended!