On the blocking board. I was scared to try it on in case it didn't fit. I wanted a snug, curve-enhancing 'sweater girl' fit - so after the seam allowance we were in negative ease territory and I was terrified it was going to be too small.
If I'd followed the pattern exactly, that band of raglan ribbing would be two bands: same length but half the width. There would be a seam up the centre of the band, joining them together. I did not fancy knitting 8 thin strips and sewing those centre seams, so I made four strips twice the thickness instead. They stitched neatly into the armpit, no problem.