Before you hypnotize or are hypnotized, you need to trust your partner to know what you want, and what they want, and have worked out your expectations.
Sit down with your partner and talk things out. This is about your general framework about hypnosis, rather than what you’re going to do in an individual session. You should have an idea of when and where you’re doing this, how often, and what your parameters are.
You need to negotiate consent, establish limits, and determine aftercare.
At the same time, beginners can’t ask the right consent questions and may not know what to expect. You don’t know what is okay for you and what isn’t if you’ve never experienced it. This is another reason to go slow, get lots of feedback, and be clear that it’s okay to stop or pause if you’re uncomfortable. Here’s a sample list showing things you can talk about. Here’s a NSFW list.
There may be specific issues that could come up that have unexpected connotations. Your partner could have a phobia of spiders, or may react badly to sudden noises. Go over things that you should avoid in sessions.
Aftercare is very individual, and depends on your relationship with your partner. Some people want to be held, other people need time to themselves to collect their thoughts, and some people want positive statements and reassurances. You should negotiate what kind of aftercare your partner will want.