Well, this is kind of a tricky question. It's just all about trying to apply the paint fast enough not to let it dry before I cover the entire area, and carefully enough not to cross the pencil lines. Which is still quite difficult to me and, as I suppose, comes with practise. Preparing the needed mix of watercolours in the palette beforehand helps save the time, though. Also, painting the lighter shades first and then the darker ones makes less mess and allows you to sweep some mistakes under the carpet. You can also use art masking fluid, it is really helpful if you want to paint a particularly large part of your work with seamless wash without overpainting small details. I have a Daler Rowney one, and it would be perfect if it didn't leave orange marks after removing from paper.
I hope this rambling might somehow help. Don't hesitate to ask anything again, as I'm sure I did a terrible job at explaining. oтz Good luck with your watercolours!