To start dealing with an ambiguous join, you create a typical multi-source query.
Choose the tables that participate in one of the joins — it doesn't matter which one — and then remove the other table from the query.
When you add fields to your query, make sure you include a field that enables you to join this initial query with your final query. The picture shows an example. The query includes the PurchaseOrderID field, even though the PurchaseOrders table isn't in the query. Instead, you'll use that field to join your initial query with your final query, and you'll see how that works in the next section, and in the practice.
As you go, remember that building a query from another query is more than a way to resolve ambiguous joins. You can use the techniques you learn here to break any complex querying problem into manageable pieces.