A piece of tubular knitting approximately the same length as the tube is wide. You could cut this from an existing article, or knit a piece just as you would a sock leg.
Some toy stuffing, wool waste or similar stuffing material.
A large needle.
A small amount of contrast yarn for the stalk and a pair of double pointed knitting needles in a suitable size for this (or you could crochet or lucet a stalk, be creative!)