The body and sleeves are worked back and forth and down top separately, and then join them together by sewing.
A video for your reference of “how to knit a perfect popcorn” has been uploaded to Instagram: jjloveknitting.

This pattern includes direction for sizes of 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, 4 markers, 5-6 nos. of 15mm button