When you don't know what to do with a spare pair of sweater sleeves...make easy socks!
The good thing is that the cuff is already done.
So, place a sock you usually use over the sleeve and trace around it. You can see that my yellow sock is folded . That is because I wanted my blue sock to be short to the ankle. You can choose the length you prefer.
Cut around the traced line and use the piece to cut the other sock.
Sew or serge around... and you are done! Takes about 10 minutes. :-)
Easy peasy, hehe. And cosy warm. :-D