Your profile features a tab titled Services, which is where you can list specific writing services that you are able to offer clients. You can add new services with the green plus sign icon, and you can (and should!) make each service description your own by clicking on the existing text to make your edits.
Basic members can negotiate rates, payment method, and other details in private with each client. If you're a Pro member, you can connect a Stripe account and sell the service directly from your profile and the Writers Marketplace.