Ask for samples, a client list, and references. If your project is critical, contact the references. Take time to look at the samples. Ask the writers how they would execute your project. Ask the writers about their fees.
Independent writers have different ways of charging for services.
Most charge by the hour or by the project, some are paid by the day or week, some by the page, and, in long-term relationships, some are paid a monthly retainer. Some charge different fees depending on the service they're providing. You should know what you have to spend. Bear in mind that an hourly rate by itself is not a measure of total cost. Some writers work faster than others.