My husband and I obtained guardianship of our daughter, then age 19 or 20 (she’s now in her late 30s). We hired an attorney to help us. I don’t recall all the steps, but a guardian ad litem sort of person came to the house to interview her–she was asked whether or not she wanted this to happen and why. Luckily for us, she was entirely willing. She understands she needs a lot of help with financial and medical decisions, and it’s a whole lot easier for us this way. We all later went to Probate Court, where the judge questioned us closely. He was reluctant to agree, since our daughter always comes across as far more “together” than she is, but the attorney was persuasive, and we became her guardians.
I think that if you’re dealing with a person who does not want the guardianship it would be very helpful first to get a neuropsychological evaluation, which would demonstrate the individual’s ability (or lack) to do things that require executive skills (planning, organizing, following through a task to completion), memory, and so on, and also to confirm the psychiatric diagnosis.
As others are noting, guardianship gives you the ability to manage a person’s affairs and confer with their providers (PCP, psychiatrist, staff if they’re hospitalized, therapist, etc.) without having to constantly get new releases signed, but it doesn’t permit you to force treatment on them. It’s important to tread lightly, since if the person under guardianship becomes irate they can always get their own attorney and take you back to court to have your legal powers removed!