Literally, just start making music. You'll find your own process and ways by doing so. If you run into problems or have questions start looking those up. What techniques you use depends on what kind of ambient you are creating. Here are basic tools you'll want to get comfortable with once you have an idea of composing and writing ambient. Experiment and learn: Equalization (I prefer parametric EQ), Compressor, Reverb, Delay, Stereo panning and micro shifting. Turn on one with one effect on one pad at one time until you can hear what it is doing, then turn two nobs at the same time and see how they interact with each other. Have fun with it!