(Devarim, Ibid.) "shall you make": and not from what is already made. He may not take threads from the garment and make tzitzith of them. "on the four corners of your garment": to exclude a garment of three, five, six, seven, or eight corners. "corners" to exclude yaga, tichla, tavletikim, takarkim, sadasim, and laburdisim, which are not squared.
R. Elliezer b. Yaakov says: Whence is it derived that one should not place tzitzith in the middle of the garment, but on the edge? From "on the four corners of your garment."