There are several legal requirements for a power of attorney to be valid. Of course, it must be signed and dated. It is expected that the power of attorney is notarized, but it can be witnessed by two people (however a bank or other financial institution might reject it anyway). California has a Statutory Form Power of Attorney.
If your loved one is diagnosed with dementia, many will assume that they can’t sign a Power of Attorney and the document will often be rejected by a Title Company when trying to get a mortgage (especially a reverse mortgage), when a loved one is diagnosed with dementia. The law presumes capacity, but many worry about elder abuse.
Banks may also want you to complete their form power of attorney at the bank. As a Certified Elder Law Attorney, I have experience evaluating capacity. I have written letters on behalf of my clients establishing their capacity to sign a Power of Attorney.
The best way to make sure your power of attorney is legally valid is to consult with an attorney.