First of all, decide what published illustrators you like. Look at your favorite children’s books and find similarities in style or coloring process that appeal to you. Chances are you probably have a few famous illustrators in mind already. This is a good exercise to also figure out what styles you really don’t like.
Next, decide which style best suits your manuscript. Think of the intended audience and format and of what would most appeal to them. Perhaps your personal favorite isn’t suited for the intended target audience of your manuscript.
Although no two illustrators are the same, you can narrow down your search significantly by writing down certain categories or criteria such as: loose, cartoony, whimsical, realistic, etc.
There are a lot of illustrators out there, picking one that can capture your vision artistically is the challenge.
Tastes are subjective, but a good illustrator should be able to show consistency when telling a story visually and be able to convey different emotions through line and color.
A good illustrator should be familiar with visual storytelling and be able to provide a lot of samples of different characters and settings. The images should not only capture the characters and the actions but bring personality and life to the sequences. The illustrator should be able to uniquely interpret your story using different perspectives from page to page, focusing on different views of the main characters and show consistency throughout.
A good illustrator cares about every small detail and should be able to use a wide variety of elements to give your characters charm and personality.
Always ask to see samples of the illustrator’s work, especially sequential stories he has illustrated to see how they handle working with the same character from different perspectives.
A good illustrator isn’t necessarily expensive, however, you can probably find some people willing to work for dirt cheap. Although there are “starving artists” out there that might be willing to work for very little, it’s a gamble. If price is your main criteria in selecting an illustrator, then quality shouldn’t be a major concern.
Indications of a bad illustrator might be:
Bad drawing skills
Bad color palette
Inexperience using medium
unsure hand and line quality
Static, stiff movement of characters