Choose an object with a simple shape, like a bottle, or salt shaker. Set it up with a single light source and a plain background. Paint it once or twice in monochrome, with an eye toward discovering which are the essential elements of the subject. Can you leave out the subtle middle values and still get the gist of it? What about the lightest lights? The darkest darks? Where is it important to get the drawing correct? Where can you let go of accuracy?
When you feel that you are starting to know the subject as a sequence of layers, introduce color into the exercise.
For each layer, ask the same questions:
Is there a way to paint the whole shape with an overall shape that can underlie everything that will come later?
Is there anything you should paint around?
Is there anything you should do while it is still wet?
Paint it a few times. As you progress, you will probably see that the range of what will "do just fine" keeps widening. Eventually, you will know the subject well enough in the language of watercolor that you can put the actual object back in the cupboard and still paint a perfectly fine version of it.
Gerhardt Richter Apfel