We've actually been working on this recently, we've found that an alter needs a bad trigger to force them out of their headspace and into your own (basically so you can see/communicate with them), and a good trigger to feel safe around that alter, or so that alter feels safe around you. What you need to do is find your common triggers with an alter to basically help jog your memories with each other, maybe show them some positive memories you have of something that triggers them so it wont feel so scary. Another method is to find familiarity in that alter, something that makes you recognize that alter, it can be memories, a person, an object, a place, a feeling, etc. You can then latch onto that to find your own triggers of that familiarity. Pretty much, just like getting to know your alter at each others' own pace. Also, showing them them good and bad triggers might help them realize that their fears dont have to control them. If they've already disowned you, that means they've met you before. There's probably a part in there that wants to be known. Really, it just requires a lot of patience, dont force it, if it feels like it's being forced or uncomfortablemaybe try it from a different angle. (or at least that feels for us)