Creation of freight invoice

Hi ,

I am a technical consultant. but wnated to clear some doubt abt the creation or freight invoice.

firstly , this freight invoices are created against the freight cost for shipping the goods if i am not wrong.

secondly when I create the invoice for this cost in Tcode MIRO how do i go about doing this as in do i do it on the basis of PO as i only have a PO as a base MM doc.

thirdly , If i do this based on the PO there where are these costs manintained in the PO and how are these picked while invoice creation

Thanks for the help in advance

Nikhil