NOTE: you should buy nylon brush tip watercolor pens. This technique works very poorly if at all with hard felt tip watercolor pens, so in my opinion, just don't buy those. Always go for nylon brush tips.
The first technique you should learn to get started, is diluting the brush tip! Undiluted you'll notice that the color is quite strong, and it layers awkwardly. Simply submerging the tip works somewhat, but you'll get best results by tapping the tip against a surface underwater. If you don't have a palette like pictured, just a bit of water on a shallow plate will work just as well.
After dilution, not only will you have less pigment on the tip, but also more water, so your colors will be much gentler, and will blend nicely, hiding individual brush strokes. Notice that the paint will be continuously flowing out from the pen, so as you use the brush the color will slowly return to its normal value, so if you are looking for a perfectly even color, you need to re-dilute the tip between brush strokes, almost like reapplying ink to a pen.
Of course, another way is to lean into the effect, and use the nice gradients it gives you.
