How do I become a health care provider for my grandparents?

Power of Attorney. If you mean the power of attorney, poa, you will need your grandparents permission. If they are incapacitated mentally such as dementia, they will need a to perform a competency exam and have an attorney submit it to the court for approval. Some states want 2 doctors to agree and some only one.
Proxy. Do you mean health cae "proxy" ? Generally the patient is the one to designate the proxy- or the person that will make health decisions for a person if they are unable to make it themselves.If they are incapable of designating a proxy, then if you are next of kin you can then become the health care proxy of record by law, .