ordering method; how to order
Refers to the way or procedure for placing an order, such as online, by phone, or in a store.
The ordering method on this site is very easy.
Please tell me how to order by phone.
注文 means 'order' itself, while 注文方法 specifically refers to the method or procedure of ordering.
Compound of 注文 (order) and 方法 (method).