Firstly, you will want to create a new email template specifically for the purpose of emailing these statements out.
Next, under Messaging Centre > Invoice template, click Choose to select your Partner Statement email template.
Click Save, and then after the window closes, scroll down below the Choose buttons to click Save again on the main page.
Now you have the emails side of things set up, let's look at the report.
Go to Finance > Partner statements.
The default view allows you to pick a start date and Search for all partner invoices within the period from the start date to today. You can filter to show the invoices that have an outstanding balance only, and also those from a specific partner.
The "Age" column shows the number of days since the invoice was closed (usually an invoice should be closed when it is sent to the client).
When you filter based on a specific Partner, two new buttons appear, allowing you to create a pdf statement which can be printed, or emailed to the partner (assuming you have set an email address in the Partners > Partner Settings screen).
The statement uses the same logo as is configured on the Messaging Centre > Invoice template screen.