Watercolor sketch during lesson...
My first advices are; not to draw too small and not to forget to use the negative space (surrounding spaces of subject and surrounding the details like the flowers and leaves) just as much as you use the subject itself. The negativ space should be interesting before you start to paint. When you have composed the subject on the paper; watch out so that it is not situated in the middle with an equally looking space around it. Also let the subject go "out of the page" so that we don't get the impression that you cut of some of it, to avoid the borders of the paper.
Specially for flowers look at the "outside contour" instead of the flower itself. Leave whites out, start with light and mid colors, but put in some dark quickly to get the contrasts fast so that you know your range of values and colors. It's easier to continue this way; as you don't have to "cover it all up" with light colors as we could tend to do, when the theory say we "should start light and bild up with darker".
You can let the brush do the shapes, just press it against the paper and the shape o fa leave will show up, let the pigments react leaving the shape alone, (don't go back and fuss, unless you are really not happy with what you just did). You can use round brushes, but also square ones for the vase here I liked using a square brush for the geometrical shapes. I also use synthetical brushes, they contain less water which avoids drowning the surface in water like we can do in the beginning.
Hoping this can be of a help... but of course, also draw first, to understand the shapes.
Then, eventually, we can paint loosely and freely...