power of attorney WA state procedure of setting up a POA

Washington Law Help has much useful information on Guardianships and Powers of Attorneys. I have included a link to their website.

You don't. Someone can only grant a power of attorney when they have the legal capacity to do so. You most likely need a guardianship at this point.

Guardianship requires going to court and can be costly. It deprives an adult of very significant personal rights. Our state’s guardianship law requires considering alternatives first.
Start by identifying the specific problems. Then consider whether alternatives to guardianship can address these problems. For example, guardianship is not a way to force someone who is competent to do what someone else thinks he or she should do. An adult who has the legal capacity to manage his or her own affairs has the right to make decisions other people may view as unwise. However, if she had capacity she could sign a POA and we wouldn't be having this discussion.

Guardianships are granted only when a person is legally “incapacitated.” This generally means the person has a mental incapacity causing a significant risk of harm. This incapacity must be evidenced by a demonstrated inability to manage property or financial affairs, or a demonstrated inability to adequately provide or arrange for one’s nutrition, health, housing or physical safety. Advanced age, medical diagnosis, eccentricity, and poverty are not sufficient to justify guardianship.

I agree with Mr. Selden. It's too late now for the POA given the lack of capacity. You will need to go to court to be appointed guardian or conservator. This is indeed a costly and time consuming process especially if you are going to be handling her financial affairs. Best get a lawyer to help.