How do I create a monthly statement to email out to customers?

It's good to see you here today, khouan. I see how customer's statement will benefit your business inside QuickBooks Online (QBO). Let me help you with this. 

You can send customers' statements when they have pending balances or invoices that will be overdue soon. Follow the steps below to begin with.

1. Go to Customers, then choose the customer you want to make a statement. 
2. Select Statement in the New Transaction dropdown. 
3. Check the customer's email, and click Save and close. 
4. Choose Print or Preview to download your customer's statement. 

For creating multiple statements and customizing, visit this link as your reference: Create and send customer statements in QuickBooks Online. 

However, if you wish to send automatic statements every month, note that it is only available in QBO Advanced. Click this article to guide you on how: Use workflows in QuickBooks Online Advanced to send reminders. 

On the other hand, check out this link to help you record invoice payments in QBO. 

For more queries regarding sending statements in QBO, reply to this thread. I'm always here to help you. 

@KayePe  RE: I see how customer's statement will benefit your business inside QuickBooks Online (QBO).

What is that supposed to mean?  The customer's business is not inside QuickBooks Online. That makes no sense.