+1
You might also want to try Chartpak / AD markers at the beginning. They are very flowing and if you just scribble inside of a shape, the ink will blend after a minute and you won’t be able to see the strokes … great watercolor-esque effect. That being said, it’s good to learn how to work with strokes too. In either case, these kind of sketches often have more life if you don’t worry about coloring perfectly inside the lines … think “loose.”
It can help to mimic an existing style before you get your own. With the image below by Syd Mead, it’s pretty simple actually – just blocks of shading. IMO certain forms like this might lend themselves better to marker sketching rather than something like a pebble.
