I have an idea to add new functions to the mobile phone. How do I apply for a patent?

First of all, you should keep your thoughts secret and don't let anyone know. If you can make a product in the factory, you should apply for a patent immediately after you make it yourself. If it's just an idea, it won't be easy without substantial object expression. You can only sign a confidentiality agreement with the partner or manufacturer, or find our agency to sign a confidentiality agreement before testing whether it can meet the patent requirements. I hope I can help you!