How to add signature in gmail
Step:-1 Click the Settings gear in your Gmail toolbar Then Click Setting Tab
Step 2 :- Select Settings from the menu that will have appeared.then Go to General.
Step 3:- Make sure the desired account is selected under Signature:.Type the desired signature in the text field.It's best to keep your signature to about five lines of text.
Step 4. Click Save Changes.
Step 5:-Gmail will now insert the signature automatically when you compose a message. Of course, you can edit or remove it before clicking Send.
For more detail check these video