What is the process to register power of attorney?

Write down the Power of Attorney (POA) terms on a piece of paper. Normally, you would do it on a INR 100 stamp paper. At this point the POA is termed as simple POA.

Reach out to the office of Registrar (commonly known as office of sub-registrar) for your residential location.
Each region within your city may have different offices.

Office of registrar would need 2 witness to sign the POA in front of the registrar (Physical presence is required).                                                                                                              

Registrar will keep a copy of POA in government records and provide one to you with his office registry stamp.