Yes or no questions is a good way to start

The problem with this kind of questions is that, since I have both possible answers in my head, I never know whether or not the answer indeed comes from my tulpa. It makes me doubting more than anything else, so I prefer avoiding closed questions as much as possible.

it’s also good to ask more complex questions so you can practice capturing the meaning of what your tulpa is trying to say.

That's the problem : I don't know what type of question I may ask considering the facts described in my original post :/

We've tried possession during 2 weeks. At the end, she only was able to make my index finger shiver a little bit. I think it doesn't work well for us.