chorale How do I write a harmonic analysis of a chorale?

Dear composers,

I have to write a harmonic analysis of Bach's BWV 4 (Christ lag in Todesbanden) - Versus 7 for my homework

Here is the link: https://www.youtube.com/watch?v=ubWCRXefVTw

I have knowledge of harmony and chord analysis with Roman numerals. How can I write a harmonic analysis?

Could you please give me some tips? Thanks in advance,

Duo