I do a lot of research and find out what other consultants are charging for similar work. Some fields charge hourly, some charge per project. It's hard to say not knowing the field you're in, but ask others in your field about what they do. When you're first starting out, people are more willing to take a new consultant if you're charging at the lower end of the range. But if you're too cheap, they'll think you won't know what you're doing.
Consultants can be internal and external.
Internal consultants work for companies. For example, in my company we have somebody who comes around and fixes our PCs when they're broken. Those people are internal. They work for the company, but they consult with other people in the company. We also have external consultants who work for another company who come in and help us with projects.