Now for the neck hole. Take the body and with it still folded for a shoulder seam, fold the fabric in half lengthwise. On the corner where the two folds meet, you will measure out half the distance of measurement (a) along the shoulder fold. Measuring down from the same corner along the side fold measure out half the distance of measurement (b) along the fold. Now connect the two dots.
Finish your neck hole in whatever manner you like. a) Keyhole; b) With facing; c) Turn edge and stitch; d) Etc.