CREATE TABLE dbo.Invoices
(
  InvoiceID int PRIMARY KEY,
  CustomerID int, -- FOREIGN KEY REFERENCES dbo.Customers...
  InvoiceDate date,
  ...
);

CREATE TABLE dbo.InvoiceDetails
(
  InvoiceDetailID int PRIMARY KEY,
  InvoiceID int NOT NULL FOREIGN KEY REFERENCES dbo.Invoices(InvoiceID),
  ProductID int, -- FOREIGN KEY REFERENCES dbo.Products...
  Quantity int,
  ...
);