Open your HTML email signature file in your web browser.
Press Cmd + A to select the entire email signature, then press Cmd + C to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Cmd + A as you may miss out on grabbing some of the code.
Open Front and click the Settings cog icon at the top right of your screen.
On the left hand side of your Settings, select the Personal pill at the top, then select Signatures from the list.
You'll notice that the page is blank, but with a message in the middle saying you have no signatures. Click the Add Signature button.
Enter a name for your email signature then click inside your signature field and press Cmd + V to paste your copied email signature.
Click the Inbox icon at the top left of your screen, then click Compose. Your email signature will now automatically insert when sending emails with Front for macOS.