Struggling with Order/Order-Detail subform despite links MS-Access - ms-access

Question

As vanilla as you can get: A main form with the order details (i.e cust number, delivery address, etc) and a subform with the actual details of the order (product code, quantity, etc). So far, so generic, and I'm really embarrassed to be this stuck.
Right now, with this setup, I'm in one of two scenarios - either:
getting duplicate order records: The first one (when I open the form) is created and links to the customer ID no problem (using rs.AddNew). However, when the form finishes populating the bound fields it shows a new order number in the order number control (that's not created yet, as far as I can see from refreshing the table). Then when I use the subform to add the order details, it creates the links to this new order number, which doesn't have the customer number in it.
If I remove the .AddNew to the recordset in my form_load, I end up not creating a new Order record (and the order number control shows "(New)"), but when I start to add order details, I then get told that I haven't given the order number in my order detail entry.
I suspect I'm entirely confused and have too many queries/too much code in too many places.
Standard table setup:Customer, Orders, Order_Detail, Products.
Order_Detail has an autonumber PK, but FKs to Orders and Products, as you'd expect.
I've got a main form that has a query that returns all the data that should go on the form. For the time being, most of the form is unbound, because I was struggling so badly with this. I can change that easily enough though.
The subform has the parent/child link set, and so I'm just at a loss.
I recognise that I'm doing something stupid: When I do the .AddNew, it's all good. When I go back and populate the form with bound values it creates a new order. If I don't populate the controls, I get (new) and none of it works.
Main form record source:
SELECT Orders.ID, Orders.Customer_Id, Orders.Display_Order_Id, Orders.Date_Ordered, Orders.Notes, Orders.Deliver_Date, Orders.Delivery_Address, Orders.Web_Order_Id, Orders.Order_Cost, Orders.Discount_Amount, Orders.Delivery_Title, Orders.Delivery_First_Name, Orders.Delivery_Surname, Orders.Billing_Address, Orders.EmplID, Orders.SpecificDelivery, Orders.CallRequired FROM Orders;
(If I remove this, it asks for ID when the form launches, so...)
Master: ID (from the Orders table)
Child: Order_ID (from the Order_Detail table)
Sfrm query (I've reduced this to be as simple as possible - it's essentially select * from order_detail)
SELECT Order_Detail.ID, Order_Detail.Order_Id, Order_Detail.Product_Id, Order_Detail.UnitPrice, Order_Detail.Quantity, Order_Detail.Discount, Order_Detail.Last_Updt_Tstamp, Order_Detail.UserID
FROM Order_Detail;
All I want to have happen is create a new order for the customer number that potentially allows it to have no Order Detail, but when it does have order detail, it doesn't throw an error.