I want to extend the sign up form of my devise installation. I created a Profile model and am asking myself now, how can I add specific data of the form to this model. Where
It's not very clear from your question, but I'm assuming your Devise model is User and you created another model Profile that belongs to user.
You'll need to create a controller for your User model with rails g controller users.
You'll also need to generate the views for your users with rails generate devise:views
so that the user can add profile info when he's creating his account.
From there, it's just like any other model: create a user and profile instance and link the two. Then, in controllers, use current_user.profile to access the current user's profile.
Note that if you're going to manage users this way, you'll need to remove the :registerable module from the User model (also read https://github.com/plataformatec/devise/wiki/How-To:-Manage-users-through-a-CRUD-interface)