If you are in Mexico, the notary will prepare and certify the Power of Attorney for you. You will need to provide the notary with your legal representative’s full name as per his/her official ID. The legal representative does not need to accompany you at the notary’s office; however, in order to be able to sign on your behalf going forward, the legal representative will have to bring his/her ID and a proof of address (for instance an electric bill).
If you are US citizen who wishes to obtain a Power of Attorney while being the US, you will need a Mexican notary to prepare the text of the Power of Attorney. As the first step, you will need to go with the Power of Attorney to a notary in your area so that the notary can certify your signature. As the second step, you will need to send or submit in person (depending on the rules of the state you live in) the certified Power of Attorney in order to obtain a so-called Apostil stamp on it.
If you are a Canadian citizen, you will need a Mexican notary to prepare the text of the Power of Attorney, and subsequently you will need to process it through a Mexican Consulate that services your area.