The sweater is worked in the round form the top down.Neck edge and yoke are worked first,then the work is split into sleeves and body ,which are then finished separately.
Cast on use the invisible cast-on . When the body and sleeves are finished , remove the waste yarn, Continue to work until the the length is needed, bind off the collar. I usually work like this . Do so not only collar flexibility, and the size of the length can be freely adjusted .