It can take a lot of effort drawing legendary Pokemon because they have such complicated designs, but with some practice anyone is able to achieve it! As for my way, first go find your Pokemon of choice-clearly published reference pictures.Some guide sketches lightly making a grid on the basis of your reference picture will help you control the proportions and balance of your drawing.
The next step is to block in the basic shapes that make up the Pokemon-think of it as drawing Reshiram's body, for instance, as an oval which has been stretched out, or Dialga's face like some kind large trapezoid.With your basic framework finished, add details in order of complexity.
But don't let your work become "dirty and coarse".Later, finish up by erasing unnecessary guide lines, and adding the finishing touches of shading and color. Use light tones first, then move onto darker if necessary to give your picture a three-dimensional effect. With practice you will improve.