Note: Make sure you are using either Google Chrome or Firefox during the installation process.
Create your email signature on NEWOLDSTAMP.com and follow these simple steps to install it in ProtonMail.
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 Gmail icon in the list of email clients.
Under the Signature preview click the Copy signature button, which will save the signature code in the clipboard.
Go to your ProtonMail.com, and in the upper right corner click Settings and in your Account tab, the Signature part will appear:
Paste your signature into the box (Ctrl+V or Command+V). Click Save:
Congratulations! Now you will see your new email signature in every email you send with ProtonMail.