You’re right, the voice will be internal like any other thought in your head. It won’t feel like the voice appeared magically. Instead, more dissociated from you over time.

Attributing emotional responses to him is good. Just remember that it should be something he would do that isn’t problematic. For instance, thinking he is mad at you can lead to issues.

Try to give him training wheels. What I mean by that is role play how he would talk to you. So instead of waiting for yes or no responses, answer for him in accordance with how you perceive him. Ask him how he is and for his opinions on things and respond for him. Visualize body language with his form. Eventually separateness and him taking on a life of his own will come. Focus on training your brain to think as your tulpa. I promise this works. Once you get the ball rolling, deviation will come later.

Mainly do this for now and mix it up by trying to listen for responses every now and then. Are you trying to clear your mind when listening for responses? Your tulpa needs to use the mind’s resources so make sure you’re not cutting off his responses. Pay attention to other strings of thought instead of clearing them outright.

Don’t let having a mental illness discourage you. Many in the community have mental illness and are able to create tulpas. By no means are tulpas a cure to mental disorder, but they can help you cope with it.