On arrival in India, you can buy a SIM card at the international airport from the kiosks of mobile service providers such as Airtel or Reliance, which are located almost immediately after you clear customs. Alternatively, you can also buy the SIM card from the stores of the cell phone vendors in the downtown areas, including shopping malls. The procedure for activation is similar with both options.
If you have the required documentation handy, all you have to do is fill up a form and submit it to the vendor. Once the documents are verified, you can purchase the SIM immediately (approximate costs Rs.550 or $10) and will get a 10-digit mobile number. Usually, the SIM is activated within 24 hours, after a confirmation call is made to your mobile phone for verifying the details provided in your application.
If you plan on traveling to several locations in India, be sure to get roaming activated on your SIM. Otherwise, you will have to buy separate SIM cards at every new place that you visit.
As per new regulations, a SIM is valid only for a three-month period after which it will have to be reactivated.