Order Entry subform

Ok - I am going around in circles with this one! I know it has probably been
answer a number of times and there are a few examples out there but I just
can't seem to get it to work on my database.... this might be a bit long
winded for a very simple answer but here goes.....
I am trying to create an order entry subform to input orders for my
customers. I have:
CustomerTable (ID, FirstName, Surname etc)
ProductCategoryTable (CategoryID, CategoryName)
ProductSizeTable (SizeID, SizeName)
ProductsTable (ProductID, Category, Size, ProductName, UnitPrice)
OrderTable (OrderID, OrderDate, OrderTime, TotalPrice etc)
OrderdetailsTable (OrderID, CategoryID, ProductSize, ProductName, Quantity,
UnitPrice, etc)

Do I need to use a query to create the subform from?
I want to use combo boxes to select category, then size then product and for
it to then populate the price etc.
I am struggling to get the combo boxes to display the correct information
from the correct source.

Do I use the CategoryTable for the cboCatergory control source or the
ProductsTable, or the OrderDetailsTable?

How do I synchronise the combo boxes? I can't get 2 combo boxes to
synchronise let alone 3!

And then how does it all get saved back to the OrdersTable and then linked
to the Customer?

Ahhh!

Any help is much appreciated
Thanks,
Rachel