What were Israelites to put on each tassel on the corners of their garments?

Answer: A blue cord

Tassels with a blue cord were to remind Israel to remember and obey all the commandments of the LORD.

““Speak to the Israelites and tell them that throughout the generations to come they are to make for themselves tassels for the corners of their garments, with a blue cord on each tassel.”— Numbers 15:38 (BSB)