How watercolour fantasy illustration is made
Fantasy illustration and watercolour have gone together for a long time, and the reason is practical rather than sentimental. Watercolour is transparent, so light appears to come from the paper rather than from the paint, and that suits forests, weather, distance and anything meant to look old. In comics and illustrated books it also lets line work stay visible underneath the colour, which keeps a drawing readable at small sizes.
Before any paint
The work begins with thumbnails, small and quick, where the composition is settled: where the eye enters, what it lands on, what stays dark. A page of thumbnails the size of a postage stamp answers questions that are expensive to answer later. Only then comes the drawing at full size, and only then a decision about the line: pencil left in place, ink applied with a pen or a brush, or waterproof line drawn and then flooded with colour. Ink that is not waterproof will bleed into the washes, which is sometimes exactly what an illustrator wants and is otherwise a disaster.
Paper, and stretching it
Paper does more for a watercolour than the paint does. Heavy cotton paper takes repeated wetting without breaking down, and its surface decides how the pigment settles: rough for texture and broken edges, smooth for detail and fine line. Lighter paper needs stretching, which means wetting it, taping or stapling it to a board and letting it dry taut, otherwise it cockles as soon as a wash goes on and the pools run to the wrong places. Illustrators working to a deadline stretch paper in batches.
Building up in layers
Watercolour is worked from light to dark, because there is no white paint to rescue you. The lightest wash goes over most of the image, and each subsequent layer is smaller and darker, laid on dry paper for a crisp edge or damp paper for a soft one. Shadows are built from several transparent layers rather than mixed to the right darkness in one go, which is what gives the depth that opaque media achieve with a single stroke. Knowing when to stop is the whole skill: one layer too many turns air into mud.
Keeping the whites
Highlights are planned, not added. Untouched paper is the brightest thing in the picture, and it has to be protected from the first wash onwards, either by painting around it or by masking it. Small sparkles can be lifted with a damp brush or scratched out at the end, and gouache or opaque white will sit on top for a final glint, but the picture always looks better when most of its light is bare paper.
Colour and mood
A limited palette holds a long sequence of pictures together better than a wide one. Many illustrators work out a colour key for a whole book, so that a chapter has a temperature and a change of scene is felt rather than announced, and they keep a few colours for the moments that must stand out. Restraint reads as atmosphere. Everything at full saturation reads as noise.
Printing, and learning the tradition
Original watercolour is lighter and more delicate than any reproduction, and work destined for print is usually painted a little stronger than looks right on the desk, then scanned at high resolution and proofed. Beyond that, the useful education is copying: take an illustrated edition of a classic fantasy you admire, work out which parts are line and which are wash, and try to rebuild one picture from thumbnail to final layer. You will learn more from one honest copy than from a shelf of technique books.