Do you still need to write the principle of patent when applying for a patent?

You should be talking about instructions. The application will be accompanied by a manual, so that you can explain your principle with your attached drawings, so that comrades in the patent office can understand your meaning at a glance. That is, the patent law says that professionals can understand it. If the patent is not very complicated, you can do it yourself. No agent is needed. I wrote all mine by myself. The design class is not very strict with this requirement. But it's easy to say.