If you have an existing AP mailbox where suppliers send invoices, you can just forward the invoices to ProcureDesk and ProcureDesk would automatically index the invoices for you.
Here are the steps
1. Let's say you have a mailbox set up only for receiving invoices. For example firstname.lastname@example.org
2. Setup auto-forwarding of emails from your current mailbox to email@example.com
3. If you don't know the exact steps for auto-forwarding the emails, Just Google it. For example "auto forwarding Outlook"
4. If your auto-forwarding needs confirmation from ProcureDesk, just send an email to your ProcureDesk Account manager.
5. Now login to ProcureDesk as Admin and navigate to Configuration -> Company settings -> Email and add your email id under "Invoices From email"
6. That's all you need to do. Every time a new invoice shows up, ProcureDesk will automatically create a new invoice with the invoice document. The invoice will always in Draft so that you can enter missing information and then submit it.