Prerequisite: Deciding what should be drawn
Artists can be found in a many places. Here are some places to look at:
Pick the best artist for the commission idea and budget. Note that some artists cannot or are not willing to draw certain content, such as nudity.
Check if the artist is available for commissioning, and find their contact details. They may prefer to receive commissions via a variety of methods such as email or Twitter DMs. Some artists even have forms to fill out.
Once an artist is found, proceed to the next step: Contacting the artist