I am having a hard time figuring out how to add values from a subform together.

I have a subform called - Payment_Information

When people pay us, I record the payment in that sub form called - Amount_Received.  If they pay in full, it's just one line and I am done.  Some people make multiple payments so I might have to add multiple payments over time.

Outside the subform in a new section I have a field called - Amount_Paid.  

I am trying to figure out a way to create a workflow that every time I enter a payment from a customer, it will update the total amount they paid in the Amount_Paid field.

So if they make one payment of $500, then when I enter the $500 in the Amount_Received line, it will automatically put $500 into the Amount_Paid field.  If they make another payment done the road, I add a line and add an additional $500 payment into a new row and then the Amount_Paid updates automatically to $1,000