Cheapest way is to trade directly with AMCs - you don't need stock brokers, or any other middle men. Open folios with your favorite AMCs which have online accesses - Reliance, HDFC, Kotak, Sundaram, Franklin and many more do provide it. This needs to be done with conventional paper application at AMCs office (or CAMs for funds serviced by CAMs), provide bank information on the form, copy of PAN and KYC ack. Fill in PIN agreement. Once you receive your PIN, you are set.
-- Milind