Here’s a list to help anyone not used to commissioning, when hiring an illustrator:
What size is the space that the art work is going to fill? Do you need a landscape shaped or portrait shaped illustration?
Images that will be printed smaller work better if they are very simple, larger pictures may or may not need more detail depending on the design around them.
Is there a colour palate in the design that you need the illustration to blend well with?
How detailed are you wanting the work to be? One (simple) object or character on a white background should take the least amount of time. (But this does depend on the subject matter).
Can you tell the illustrator which of their existing images you like and which you don’t like before they begin drawing for you and why?
Deciding what you ‘don’t’ want in an image ‘before’ you commission it, saves a lot of time and examples of what you do like and don’t like help too.
“I know what I want when I see it”, is a very expensive attitude to have. The more prepared you are the more money you will save.
A good creative will give you a rough sketch based on exactly what you’ve asked for, and another option that they might personally think could work just as well or better. That way you can be open to suggestion and possibly more creative ideas whilst still having your initial thoughts visualised and all bases covered.
If you have a very specific layout or idea, don’t be afraid of drawing your own rough sketch, even if it just involves stick men. Try the rough composition that you’ve drawn in your space, see if you like it, then hand it over to the illustrator and it will give them a better idea of what you need.
Feel free to give the illustrator free reign if you don’t trust your own thoughts on a project and you are prepared to trust their experience. However, if you do this, decide to go with their ideas right at the start and then trust their judgement. Deciding to go with your own ideas after all, at what should be the end of a project, could proove expensive.