For us at least, it just happened naturally. I often talked to myself and felt as though there was definitely more than one of us, until one day our friend told us that we might be plural and we realized that we could really relate with the plural experience.
So, how did you figured it out?
We used to talk to ourselves, we thought it was a game, we were sad when we broke by asking, “who are you?” And the answer was, "you "
But one day we started smiling and saying, “I’m you! And we can go on adventures together!” And then we would journal about it to prove that i really did get along with myself.
Sadly this only happened when we were out of depression, which wasn’t very often.