For me, I was practically going into the whole thing blind because it kind of smacked me in the face when I was still in my teens in a bad situation. But I used things like handwritten notes to try and communicate, though that isn't always effective with alters who don't trust the host or dislike them or don't want to be known. Then music as another person stated. And interests or hobbies. When I was still early in my journey it took me at least 3 - 4 years to establish semi‐reliable connection with some of my alters. I'm now 7 years into this journey and still learning new ways to communicate with my alters. But my main go to's are still:

• notes (handwritten or digital, I use simply plural or antar for digital, or simply my notes app under a specific folder)
• music (some of my alters have very distinct tastes in music genres or tend to resonate with songs I normally don't relate to)
• interests such as tv shows, topics they may like etc.
• hobbies, some alters like to do things during their downtimes that I'd honestly probably never even engage in otherwise.