1. Go to Settings at the top right corner of your Gmail page
2. Click the Labs tab in order to go to Gmail Lab
3. Find Signature tweaks feature and enable it by selecting the radio button against Enable button
4. On the top of the Labs page, find the Save Changes button and click it to save the changes to the Gmail Labs feature settings you have made.
5. Thus, when you reply to any mail in your Inbox, you get the message editor window with the signature at the top as follows:-
In the inbox, open the message to which you wish to reply
Opened message would look something like below:-
When you click to Reply or forward the message, you would see your Signature at the top of the original message sent by the sender (in this example Sanjeev)
Note: When this Signature tweaks Gmail Labs feature is enabled, it also removes — appearing before the quoted text in your signature. By default Gmail places two – (–) before the quoted text block of your signature.