This is one of a special series of "school patterns" that McCall issued for beginners. After mastering other patterns in the series for a nightgown and a petticoat, the novice could move on to this iconic middy blouse.
McCall patterns at this time didn't have separate instruction sheets. Instead, the instructions were largely printed on the pattern pieces themselves. This isn't a bad approach. You quickly learn to keep the pattern pieces pinned to the fabric until you're ready to sew them, then to review the instructions on for each of the pieces you're about to sew.
For this series, however, McCall decided to print all the instructions together at the bottom of the sheet of tissue.
Look at all those pleats at the bottom of the sleeve!