For employees, the procedure of employing a temp agency is pretty easy. It's the same as applying for a job. You submit a CV (perhaps online, depending on the agency), complete an application, and then have an interview.
This interview may be relatively quick for entry-level employment; for higher-paying occupations, it can be more like a complete job interview. During the screening step, the agency may perform a background check or request a drug test.
If you are accepted into the agency's workforce, you will be given one or more positions that match your abilities, if any are currently available. There might be a wait of many days or weeks before anything becomes available.