Let me say at the outset that this is my first go at this technique, and all my information so far has come from Youtube and paint manufacturers websites.
It was enormous fun, and now that I've seen the principles at work, I can get on and do more considered pieces I hope.
Here's what I started with. I already had high flow fluid acrylics, but these aren't necessary. As I understand it, you can use practically any paint - it's the consistency that's important, so mix thicker paints with water until runny.
I also used Liquitex Pouring Medium , but I think Golden GAC 800 and other makes do the same things (no affiliation here btw!)
I poured about half paint to pouring medium into bowls, and stirred to mix.
I then poured my mixed paints, one at a time, into a small jug (jar/cup/whatever you have) but didn't mix or stir them. I then gently poured this onto a canvas. The canvas was primed with gesso but no other paint. I then just tipped the canvas to help the paint run to the edges of the canvas. That was it!
By the time the paint had run across the canvas, the colours had mingled and the results were like this:
This was ok, but I wanted something called "cells" in the paint to make the effect more interesting. I'd read that you could do this by adding WD40 to the paint mix. So I put some more mixed paint/pouring medium into the jug again in the same way, and just before I poured I gave a healthy squirt of WD40 into the jug as well. (The photo below shows paint that has "cells" in it - a bit like adding salt to watercolours).
This improved the effect a little bit, but the cells I created didn't last, and disappeared in a few minutes.
For a first try, I'm very pleased as it was enormous fun to do. I intend to try again very soon, using alcohol to create cells - come back and take a look!! Another lovely thing about doing this, is that the finish on the painting is glorious - it's like looking at stained glass.