They're great in sketchbooks and for creating watercolor effects when water is added.
If you're adding water, use watercolor paper. Lower quality paper can develop a weird texture.
Excess water can cause blooms.
If your markers get dirty, scribble until the color is correct.
Color with the darker color, then add the lighter color on top to blend.
This will create "watercolor" to paint with.
Draw with the marker and use a blending marker to soften the edges.