One of the main challenges to mastering any form of painting (or drawing, for that matter) is learning to control hard, soft, and lost edges. Once you’ve figured out how to create those three, then you’re a lot closer to mastering the medium.
In the case of watercolor, making hard edges is very easy. All you have to do is put paint on paper, and you have a hard edge. Soft edges, on the other hand, take some thought process: you have to put patches of different pigments, or a patch of pigment and a patch of clear water, next to each other and allow the two to merge on the paper to create a soft or a lost edge. If the two patches are not equally wet, it will change the mixing dynamic. There is no easy way to do this: it will take time and practice to master.
That means that you should start soft in the underpainting stage, and stay soft as long as possible, only using hard edges toward the end of the painting to pull out accents and highlights. If you do so selectively, only putting in detail and texture in your most important, focal areas, you can draw the viewer’s eye to whatever part of the painting that you want to.
As an example, here’s a painting of mine in progress. As you can see, at this stage the painting is entirely mushy. At this point, it will be easy to pull out the little bit of texture I need to make a final image.
I try to utilize soft edges to the maximum in my paintings. For an example, see Nelson Mandela, and notice that his shoulders are nearly lost, his hair and the sides of his head are soft, and things only truly clear up in his facial features.