When you add a Contact Form to a project, it means that when you are creating a new contact, you can choose this Contact Form as the one that will be filled in when creating the contact.
Steps
Go to the Project Preferences page of the project that you want to add the Contact Form to
You can get there by going to the project page. Press More Options and then select Project Preferences
In the Contact Forms to use on this project field: select the Contact Forms you want to use
Press save
You might be prompted to select one of those Contact Forms on the Add a contact form to the Enrolled state field