Note: Make sure you are using either Google Chrome or Firefox during the installation process.
Salesforce allows only 1333 characters in the signature section, making sure your signature is neat and not bulky.
After you finish creating your email signature, click Save and Install button under the signature preview.
Or if you have saved your signature priorly, hover over your signature on the Dashboard and click Use button.
Select the option Other in the list of email clients.
Select the HTML tab, scroll down the Signature code and Click the Copy and Proceed button, which will save the signature code in the clipboard.
Open Salesforce and go to your profile Settings.
Please scroll down to Email > My Email Settings and Paste the code into the placeholder.
Save the changes and check your new signature by sending a test email.
Congratulations! Now you will see your new signature in every email you send with Salesforce.