You can add a signature to an invoice in Moon Invoice easily by following the steps below.

1. Open the Moon Invoice application on your device or tap on this link.
2. From the main menu on your left, go to Sales ➜ Invoices.
3. Open the existing invoice and tap on the ‘Signature’ icon. 
4. A pop-up will appear on the screen where you can enter the name, title, date, and signature.
5. To add a signature manually, use your mouse pointer and adjust the Thickness as needed.
6. To add a signature from your device, click on the Upload button.
7. Click on ‘Done’ to apply the changes.