Scumbling isn't tricky to learn, but does take a bit of practice to do confidently. The two important things to remember are to have very little on the brush and to scumble onto dry paint.
If you've got too much paint on your brush, or the brush is very wet, when you try to scumble the paint will spread. The little gaps will fill in and you'll end up with a smooth, even area of color, which isn't what you're after when scumbling. You can see an example of this mistake in the photo, on the right-hand side of the painting.
If you scumble onto wet paint, the colors will mix (a physical mix) and ruin the effect (which creates an optical mix). Scumbling should be done onto paint that is absolutely, definitely dry. If in doubt, wait. Working onto dry paint also means that if you don't like the result, or put down too much paint, you can lift it off with a cloth. (Though if you're scumbling with acrylics, you'll need to do so very quickly!)