I have done this many times. Follow the procedure given below.
1. You go to the post office of India Post or go directly to Railway Mailing Service (RMS) office. Write the to and from address on the parcel.
2. Give the wrapped parcel to the booking clerk and tell him to do Speedpost. The booking clerk will weigh the parcel and ask you to give some amount of money respective to the parcel weight.
3. Give the money as per the weight, you can check the rates in the rate list hanged somewhere near the office.
4. The clerk will register the parcel for speed post in the computer.
5. After the registration the clerk will paste the barcode sticker (for Speedpost) and one copy of registration slip in the parcel and give other copy to you.
6. Hence, your parcel is ready for its journey.
Follow the same procedure for the private courier service.