This can be done on the Projects > Billing Options screen for a specific project. Select the project you want to assign a fee schedule to and click View Details from the row-action menu. Under the Billing Options tab, you can select one of the fee schedules to assign to the project from the drop-down. You can also assign the project to the fee schedule right from the Fee Schedules screen. Check out this article on assigning fee schedules to all projects.