That is not so much a matter of ‘configuring’ but more a matter of ‘building and creating’. You will have to set up the entities in the domain model: ‘Customer’, ‘Product’, ‘Order’, ‘Orderline’ with ‘Orderline’ having a *-1 association to ‘Orderline’, In the domainmodel, right-click ‘Customer’, and ‘Generate page’, (also select Product and Order, not Orderline) and you have your first pages. On page ‘Order_NewEdit’ add a Dataview and give it datasource ‘listen to widget’. In that DataView add a Datagrid for the Orderlines.
Now you have the basis. Still to create the routine to give the Anonymous user the option to create a Customer and switch to it. And the routine to retrieve his data from the existing sales app.
For more understanding of Mendix, check out academy.mendix.com, for instance the rapid crash course
For a great example project that is including some functionality and a showcase on what a Mendix application can accomplish: see the Base application.