When first starting out, attempting to play ambient guitar can be a daunting task, especially due to the very nature of the style, which may be described as "washy" or even "indistinct" at times. Ambient style guitar tones can be so saturated in effects that it may be difficult for the beginner's ear to distinguish rhythm and note definition. However, with only a basic understanding of effects units and playing technique, you can be well on your way to achieving those beautiful guitar tones that you have heard in your favorite recordings!
The most important thing to keep in mind while playing ambient guitar is that even though this style of play may at times sound "random", it is not. Even when playing the subtlest of parts, every player should at least have a general plan of what notes he / she will play that will compliment the chord progression of the song. Keeping this main principle in mind, ambient guitar tones can be broken down into three different aspects, which if mastered, can provide you with that tone you're looking for. The three aspects are: Size, Decay, and Layering.
"Size" refers to how big you want your guitar to sound. However, it is important to realize that size is not the same thing as volume. One of my favorite size providing pedals is the BOSS RV-5, which when switched to the "modulate" setting produces a beautifully spatial and expansive reverb sound. With the other three knobs on the pedal set around the 2 o'clock position, you get a very large yet controllable reverb tone.
"Decay" may sound grim, but this is the term most commonly used in the guitar world to describe how long your effects last. This is where things can get tricky, because the ambient style of playing calls for longer decay settings on your reverb and delay pedals, however when decay settings are too long, notes and chords can mush together and become indistinguishable from one another. Striking a balance between "dry" (no decay time) and "muddy" (too much decay time) tone is key. I personally use the hall reverb on the Strymon Flint pedal, which I feel creates a pleasantly lingering cloud of sound that does not overpower subsequent notes. In addition, I have become quite fond of a dotted-eighth note delay tapped in at quarter note speed on the Strymon Timeline delay pedal.
"Layering" is perhaps the most subjective aspect of ambient playing. Generally speaking, it is a good idea to mix in higher octave notes on top of lower register chords so as not to lose note definition in chord changes. However, this principle is completely dependent on the certain musical situation that you find yourself in. If in a full band, where the bass player is covering the lower octave, try a mix of mid register and higher register notes instead of chords. The more octave registers a band can cover the fuller the music will sound. Another beneficial layering technique is varying the tone of your guitar. Switching pickups, engaging drive pedals, or even using a tremolo effect can be a fun and useful way to spice up your tone and create multiple layers throughout a song.
Focusing on these three aspects of ambient guitar tone will help you expedite your search for those big, lush guitar tones that every ambient player is after, but always keep in mind that at the end of the day, it's all about having fun and playing things that you enjoy!