This style of mark making falls into the "controlled chaos" category of painting. The results are somewhat unpredictable, but with experience you learn to control the results to some extent. It's not hard to learn to do, but it takes a willingness to experiment and risk failure to master. Worse case scenario: you wipe it off or overpaint it and start again.
Step 1: Apply a little paint directly onto the canvas. Fluid acrylic and acrylic ink work better than tube paint because they will spread more readily when water is added. How much and where you put the paint this depends on your composition. Again, practice will teach you.
In the photo, top left, you can see a seascape I was working on, with an initial layer or underpainting done on the sea. I've masked off the green coast area with masking tape, so the paint won't spread into this area (though it did seep underneath a little as it was low-tack not high-tack tape). I've then applied quite a bit of Prussian blue acrylic ink near the horizon and less towards the bottom of the composition, using the dropper from the ink bottle to apply it both by drawing lines and splashing some drops. There's also a few drops fluid titanium white.
Step 2: Spray some water onto the paint. I use a fine mist spray produced by Atelier to go with their Interactive Acrylics (Buy Direct). It produces a very gentle mist of water, with any big drops. You may find a plant mister that gives a fine mist, or use a perfume atomizer or similar.
The distance away from the paint that you spray the water influences the results you get. Spray at arm's length, and a large area of the canvas (perhaps even all of it, depending on the size) will get wet. All the paint will spread very similarly. Spray up close, and you get a more forceful jet of water onto a small area, and you'll get a longer run of paint from a small section.
How much water you spray onto the paint, and at what distance, is again something you'll learn with practice. It's better to err on the side of too little than too much, as you can always spray on a bit more if the paint isn't running as much as you'd wished.
If you want to spray a number of small sections, laying the canvas flat while you do this and then lifting it vertical means you can focus solely on the spraying, rather than on where the paint's running and where you've still to spray.
In this painting I sprayed multiple times up close along a line on the horizon and then a little lower down, which is how I've ended up with bits of the lighter blue from the underpainting showing through in places. Likewise near the coast and at the bottom. In the central area I spray from further away, to get a more general spread of paint.
Step 3: Control the Chaos! This is the fun bit. Angle, tilt, turn the canvas and the paint will flow this way and that. Blow on a puddle of paint to encourage it in a particular direction, or pull the handle of a brush through it to encourage it to stream that way.
In the photo, bottom left, you can see the mark making the flowing of the paint has created. Most of the white has been overwhelmed by the blue, but the larger puddle of white has blended out into a soft shape.