$user = new User() in the controller and pass it to a form that lets you fill up the project details and save the record to a Project table. Here User is a model and project is declared as a relation to the user.
If a project already relates to a user I would just pass the id of the user the project needs to be related to, and then create a hidden field that will hold its user id with a name like Project[userId] (if that is the case) and then when user submits the Project form, it can just update its attributes as normally.
if you check the HTML code of the any form you will figure it out easily. You will see that field names are set in a way of ModelName[attribute]
It is just another way of doing it… without breaking your head against the wall.