Listen so we know that Sebastian didn't have many human skills like cooking etc at the start of the contract right? so maybe at the very beginning, when he first meets Nina, she teaches him the very basics of sewing, just in case he needs them..

Just imagining like,,, a few days after Ciel returns from the cult with Sebastian, Angelina contacts Nina and asks her to go and check on Ciel, make him some clothes etc bc obvs the mansion burned down with everything inside of it.

She takes all Ciel's measurements, (notices all the scratches and the brand and everything else), and then says she'll stay for a day or two to throw together a few outfits and essentials for him. So Ciel goes to take a nap and she sits down and starts working on his clothes, some shorts, some socks, a nightshirt and a few coats, just enough to last him until she can get him a proper wardrobe. She drags Sebastian along with her thinking 'well hes a butler, he has to have SOME kind of sewing knowledge, he could at least cut some cloth pieces out' but NOPE hes completely useless.

So she sits him down and starts showing him some basic stitches, just in case something happens and he needs to fix something, while asking him about everything that's happened.