Method 1: The easy way
Draw the picture in pencil on watercolor paper, paint in watercolours over the top, then when dry go back over the image doing the line work in ink / paint.
Method 2: Keeping your original, natural pencil drawing
“Often when redrawing a page, the result is never quite the same, so in order to use my rough, messy drawings for the final artwork, I had them printed onto watercolour paper using the kind of large-scale plan printer that architects use. This way the pencil lines remained bold and I could paint as freely as I wanted,” Freya Blackwood
Method 3: Tracing paper to make indentations
(Inspired by this Dick Bruna video)Put tracing paper over your watercolour paper. Draw your image, leaving indentations on the paper. Paint in the watercolours, then add lines to the coloured image.