Watercolor pencils confuse people because they look like colouring pencils and behave like paint. Shade with one and nothing much happens. Touch it with a wet brush and the colour dissolves and floods.
Michele Webber runs through the core techniques here on a single grid of test boxes, which is the sensible way to learn them - draw yourself the same grid and work down it rather than trying any of this on a painting you care about.
The one thing that causes most bad results
Not enough water. Almost every disappointing watercolor pencil wash is a pencil mark that never fully dissolved, so it sits there as a visible line under the colour. If a wash looks scruffy, the answer is usually more water and a bigger brush, not more pigment.
You need fewer pencils than you think
Because the colour mixes on the paper, a small set goes a long way. Yellow laid over blue gives you a citrus green you will not find in any tin, and it will be a better green than the one that comes ready-made.
Two habits worth building early
Shade lightly. Pressing hard dents the paper, pigment collects in the dent, and that dark line will not wash out afterwards.
Let layers dry. Going back in too soon lifts the colour underneath and the two turn muddy rather than stacking.