For this purpose, you need to create a new sale, through which you set the document type to "Advance Invoice".

After choosing the document type, you need to choose a client, the items, their quantities and prices. After that you need to fill in the advance. The transaction will remain unpaid until you add a final payment with a final invoice.

You may find your newly created advance invoice in the list with sales. The sale will be in yellow color because there is only a partial payment on the transaction. On the right, you will find a button to generate an additional invoice, which can be either a final invoice or another advance. Every invoice issued by you can also be found in "Documents" - "Invoices"

You can print out the document or send it via email.