You answered your own question! It's that simple. I did it a long time ago for pasta I think, and since I could not remember the finer points of the technique, allow me to refer you to Thomas keller's explanation.
In a nutshell, he pulses the greens (spinach and/or watercress/parsley) in a robocoupe (or push through a grinder), add 4X the volume in water and soak overnight. Strain the water in a pan, squeezing all the moisture out, and bring to a boil, stirring constantly. When it's fully separated, cool quickly with some ice and strain through a cloth overnight. This will keep for a couple of days only.
What are you using it for?