For this project you will need;
2-2,5 kg of yarn
2 sock-knitting needles in a fitting size
Step 1: Choosing the right yarn
If you cannot get hold of this kind of yarn or you are lazy you can also buy very thick wool (rather expensive) or cord and consequently skip step 2 - making a cord. The Norwegan Pickles sell a very thick yarn that can be used for a pouf; Pickles fat and happy.
You can use Zpaghetti, cut up old t-shirts yourself, or buy tube-yarn or felted cord. The last you can get
at Etsy (handmade) or as industrial grade felt i.e. at Filtmaatschappij Rotterdam (this is where Christien Meindersma got hers, I think) You can also use woven (cotton) cord i.e. from the Hema.