A power of attorney (POA) is simply legal permission for one person to act on behalf of, and in the interest of, another.
I held a power of attorney for my mother, and later for my father, who lived in one state while I lived in another, and acted under it to do business with various entities in multiple third states. There was no major problem.
The power of attorney should be drawn in the form specified or recommended by the state where the grantor (principal) lives, and notarized there. It should then be sent (mail will do) to the agent (the person named in the POA). Some states want the agent to sign a provision of the POA saying that the agent accepts the POA and will act properly under it. This may need to be separately notarized. You will need to provide copies of the POA to people or firms you want to do business with under its authority. Some of them may want you to sign a separate paper in which you state under oath that the POA is valid, and has not been revoked.
The POA should sate what matters, or categories of matters, the agent is authorized to handle. It can be limited to a single transaction, or cover all of the principal's affairs, or something between. It can include an expiration date, but usually will not. Any POA is cancelled if the principal dies.
You can have a lawyer draw up a POA, but there are software programs that will take the principal through a questionnaire and put together a POA document appropriate to the specific situation and the principal's state of residence.
If the principal has problems with travel, it may be possible to arrange for a notary to come to his or her residence to have the document singed and notarized, for an appropriate fee. The principal will probably need ID to show that s/he is the person named in the POA document.