It's notated as it's physically played, one octave below the sounding pitch. Usually indicated with "harmonics" or "harm." and terminated by "ord." or "naturale" or something like that. I'm just using Roman's convention.
I made a video on it for violinists, but not really one for composers specifically, though it's a very agile technique without many unexpected limitations in the hands of a skilled player. Unlike standard harmonics, the notes can be played extremely fast, with vibrato, in scales, trills, and so on.
They tend not to speak well above the middle of the string (so you get about an octave's worth of useful range on each string), but it's not a hard ceiling. In fact I break this ceiling slightly in the Romance myself; in m.85 there is a single pitch that must be fingered a minor ninth above the open A. The pitches in harmonics that would be associated with the open strings must be achieved on the string below, obviously. Of course there is no string below the G string, so getting the harmonic at the pitch G4 (392Hz, one octave above the open G), requires the use of the usual natural harmonic at the second partial instead of a forced harmonic, which may make some passagework a bit awkward in some cases.