1. Go to Settings at the top right corner of your Gmail page
2. Click the Labs tab in order to go to Gmail Labs
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 compose any new message, by clicking the Compose Message link at the top left of any Gmail page, you get the message editor window with modified signature as follows:-
Note: When this Signature tweaks Gmail Labs feature is enabled, it also places your Signature at the top of all the chain of messages to which either you are replying to or forwarding. By default Gmail places the Signature at the bottom of all the set of messages when you are going to reply with history or forward them.