As promised this is my Carrie Bradshaw's signature bun tutorial. It is really easy to do, you just need the "roll bun". I bought mine at Primark for £1 but I am sure that you could also find it at H&M or maybe Claire's for a quid or so.
I hope that everything is clear, if you have any question just ask. And have a great weekend !
PS : If you don't have the roll bun, you could use an old (clean) sock. Just cut the end of the sock, then roll it over, and following the same steps you should get a similar result. Hope it was helpful !