There are two ways to go about it:
From the project level navigate to (Project-level) Administration > Quality Profiles and you'll have the per-language ability to associate specific profiles with the project. You must have admin permissions on the project to do this.
From the global level navigate to Quality Profiles, click on the name of the profile you'd like to assign and use the Projects part of the interface to assign one or more projects to the profile. You must have the global Administer Quality Profiles permission to do this.