I would like to install rediffmail how can we install

In order to use rediffmail click on the following link

http://www.rediff.com/index.html

(if the page does not open copy and paste it in your internet browser like Internet explorer or firefox)
After the web page open click on the yellow envelope icon (yellow letter shape) on the screen. Click it and you will see a small screen open and the background will go blurred.
At the end of screen you will see the following message saying

New to rediffmail?
Create a rediffmail account

Click on "Create a rediffmail account"

and a form will open and fill in the form with your detail
"Remember the username and password you give, write it down on a paper" Remember that rediffmail works only for a limited number of country I hope its not an issue in your case.

Click the "Create my account"
and then click the web link again on top of this solution. the web page will open again go to the yellow letter envelope icon saying rediffmail and click it the same small window will open.
Type in the "Username" and "password" I told you to remember and write down earlier and you will be using your rediffmail. There is no installation it is online.

I hope I solved your problem.