Quit a job professionally and respectfully by submitting a letter or other formal communication to your supervisor with ample notice before your intent to leave, schedule a meeting to discuss the transfer of responsibilities for your position and agree to provide training to your replacement. It is also important to thank your employer and coworkers and share contact information to stay in touch after leaving.
Many states feature specific requirements for notifying an employer about the desire to quit a job, particularly in regards to the amount of advance warning you must provide before leaving. Before speaking to your supervisor about quitting, research your state's laws to ensure you comply with all applicable legislation. Some state may not impose restrictions on contractors, while others may offer an "at-will" status that allows for immediate quitting without reason. Regardless, it is often helpful to provide at least two weeks notice before leaving in order to allow the company to make arrangements for your responsibilities.
You should also provide a letter to your employer stating the reason you are quitting, though it does not need to go into extensive detail. Give the letter to your supervisor in person, and discuss why you are leaving so that you can maintain the work relationship and provide feedback for the company. This also allows you to plan your exit so that you have minimal obligations for your remaining tenure.