What do I need to get POA for my Mom (90) with Alzheimer's living in a nursing home?

I suspect if she can no longer sign her name that she is not mentally competent now. If she is not mentally competent, she cannot appoint a POA. You would have to get guardianship to handle her affairs. This is expensive and can take some time, so I don't know if you would want to do it. Who has been handling her affairs up until now?