Rose, the town librarian, is your first client. She pays 200 coins for a professional work dress that won't stand out. You don't need to buy anything: the cool grey linen you already own covers the professional requirement.
How to complete it
Measure Rose
Drag the measuring tape over her bust, waist and hips.
Draft the dress
In the sketchbook pick a princess bodice and add a collar to raise professional. Choose peasant sleeves and a pencil, princess or straight long skirt.
Use your grey linen
Cut the dress from the cool grey linen already in your stock. That is enough to finish the commission.
Sources: Into Indie Games commission walkthrough, full playthrough video, gameplay video, first look video, Dressmaker Wiki demo walkthrough.