IMPORTANT: Before you begin, make sure that you don’t have Plain text mode enabled. To learn how to disable it, click here.
Sign in to Gmail using Firefox browser (Chrome adds hidden things to the signature which will result in a “The signature is too long, please try a shorter signature” error. So please use Firefox)
Click the gear icon in the upper right corner and choose Settings.
On the General tab (default), scroll down to the Signature section.
Paste the copied signature in the Edit signature section (Ctrl + V).
Choose whether to insert the signature at the very bottom of the conversation chain, or under the latest replies (optional).