How do I apply a payment to an item when the payment currency and item currency are different?

How do you apply a payment to an item when the payment currency differs from the item currency? I am trying to apply a payment denominated in USD to an item denominated in Candian Dollars CAD. I tried going to the Currency Exchange Aid and the Currency Conversion Worksheet but was unsure of how to use them. The Currency Exchange Aid adds additional rows which still do not balance the payment application. The balance remaining kept changing when I tried to use the pages, so wasn’t sure how to get the payment to apply. Thanks.