Dematerialisation is the process by which physical share certificates of an investor are converted to an equivalent number of securities in electronic form and credited into the investor’s account maintained with his/her depository participant (DP). It is like having a bank account where instead of money, you hold securities in your account.
In order to dematerialise physical securities held by an investor, he has to fill in a DRF (Demat Request Form) which is available with the DP and submit the same along with physical share certificates one wishes to dematerialise. Separate DRF has to be filled for each ISIN Number. The complete process of dematerialisation is outlined below:

Surrender certificates for dematerialisation to your depository participant.
Depository participant intimates Depository (NSDL or CDSL) of the request through the system.
Depository participant submits the certificates to the registrar of the Issuer Company.
Registrar confirms the dematerialisation request from depository.
After dematerialising the certificates, Registrar updates accounts and informs depository of the completion of dematerialisation.
Depository updates its accounts and informs the depository participant.
Depository participant updates the demat account of the investor.