Help - how to re-upholster dining chair

Hi Everyone,

I am a beginner and looking on some advice on how to re-upholster these dining chairs?

The seat part seems fairly straight-forward, it's the backrest that I'm not sure how to tackle.

https://preview.redd.it/3bnyhyxzplzg1.jpg?width=628&format=pjpg&auto=webp&s=0b9dd72877a799d6b08ba7f67b6cb798fb23f563

Any advice welcome !