How to Add Mail Signature on iPhone 

Learn how to edit the default iPhone email signature across all of your email accounts in this step-by-step iOS tutorial.
An email signature is text, like your contact information or a favorite quote, that’s automatically added at the end of email messages as a footer.
Steps to add signature:
1. Navigate to Settings on the home screen of your iPhone
2. Navigate to Mail settings
3. Within Mail, scroll down to the bottom and click on Signature
4. You can then customize a signature for all your email accounts or for individual mail accounts
Let me know if you have any questions in the comments. Cheers!