Musically speaking, the most important ingredients for this style are long, tonally static drones, occasional chord progressions, and sparse yet simple melodic motifs. Also, adding the rhythmic elements, such as synth arpeggios or discrete percussion beds, can give the needed momentum though these shouldn't stand out too much. The most important thing is the delicate balance between the foreground, that should evolve in a simple and unobtrusive way - through discreet harmonic and melodic changes, and the somewhat static backgrounds involving drones, pads, and rhythms. These, however, should be addressed with production techniques, especially with automated filtering and other FX parameters as the piece progresses. So they also change through time, but not necessarily in a musical sense.
Speaking of sound sources, many things can work well, even using only acoustic and electric instruments as well as field recordings. There are also plenty of libraries suitable for ambient work, ranging from more traditional ones to the more modern concepts such as the EVOs, featured in several Spitfire Audio libraries.
