Build a wall and a door.

A sound attenuating curtain or vertical blinds would help but with that angled ceiling you'd have to have someone figure that out.

https://www.youtube.com/watch?v=ZIxYpk2POhw

It’s the angled ceiling and the architecture etc that gives me pause on just building just a wall and a door. Thought someone might have an insight thank you

It would be more accurate to say you want to turn extra space/hall lofted sitting room space into a bedroom. I see doors that open to the loft space. Putting up a wall would eat up much of the footage. What is the square footage and what does it look like if you stand where the man is and take a picture from that angle? These loft spaces look kewl at first but are seriously inefficient.

This room was used as a master bedroom previously and we plan to do so as well. This first pic is still aimed in the direction of the previous pics and the only one I have facing the other way shows the entrance to the bathroom and the door to a little deck.