The following manual describes the way to get your signature to the default Mail app on iOS for 14.0 and newer versions. If you're looking for the Gmail manual, please follow here.
The following manual may sometimes result in images not being displayed. If that is your case, please skip below to an alternative version.
Go to the User page on your iPhone or iPad
Scroll down to the Download HTML option
3. Click on it and you'll see a prompt to download the signature. Confirm it.
If this option didn't show for you but your signature popped up instead, go back and Tap and hold the orange Download HTML link. A menu will appear, choose Download linked file
4. Go to Files
5. Tap and Hold on the signature file until a menu appears:
6. Select Copy.
7. Now launch Settings from your Home screen.
8. Go to Mail.
9. Scroll all the way down to Signature
10. Delete your current signature
11. Tap and hold for a second, when you release your finger, an option to Paste will appear.
Your signature is now ready and will now appear at the bottom of emails sent from your iPhone or iPad.
Alternative version
Go to the User page on your iPhone or iPad
Click on "Signature preview"
Copy the content of your signature from the pop-up window
Go to Settings->Mail->Signature and paste the signature there
If the formatting changed, please use "Shake to undo" feature to revert it to the desired formatting