The body and sleeves are worked back and forth and down top separately, and then join them together by sewing.
A video to show how to work a perfect popcorn has been uploaded by instagram: jjloveknitting

sizes: 1-2 (2-3) 3-4 (4-5) 5-6 (6-7) 7-8 (8-9) 9-10y
needles: 3.25/3.75mm needle
notions: one sewing needle, 2 markers, 5 (5) 6 (6) 6 (6) 7 (7) 7 nos. of 15mm button