Make Rows of Machine Threads on the Upper Side
Puzzle 6 of Life of a Book Medium Pack
Question is: Make Rows of Machine Threads on the Upper Side
and the answer is: TOPSTITCH
Word Meaning:
TOPSTITCH — (sewing, uncountable) A sewing technique, most often used on garment edges such as necklines and hems, where it helps facings to stay in place and gives a crisp edge.
Definitions from Wiktionary.