Open your HTML email signature in your iOS device's browser app.
Tap and hold your finger down on the text in your email signature and you will see a tooltip appear. Tap Select All, then select Copy.
NOTE: If the tooltip does not show, you can drag the highlighted area above and below to grab the full email signature.
Open your Airmail App on your iOS device. Select the menu icon at the top left of the app, then select Settings at the bottom left.
Tap the email account you want to install your email signature into, then select Signatures.
You will now see a blank signature text field to add your email signature into. If there is any text in here, delete it. Once empty, tap and hold inside the field, then press Paste.
Your email signature is now installed and will automatically appear when composing emails from the Airmail iOS App.