You'll need to start by selecting a platform or framework for integrating the chatbot functionality. There are several options available, including using pre-built chatbot platforms like Dialogflow or building a custom solution using frameworks like TensorFlow or PyTorch. I found guide about Integrating ChatGPT at
https://yellow.systems/blog/chatgpt-integration with step-by-step instruction about integration chatbot into mobile apps. There are all info about the process of integrating ChatGPT into the mobile app, covering key steps such as platform selection, training the model, implementing NLU techniques, testing, integration, and ongoing improvement.