I don't think the instructions were written/proofread well. I'm thinking you are supposed to cut the appropriate length (knee up and over shoulder and back to other side of knee) and width (I think I'd measure maybe half way to the elbow?). Then fold in half width, then fold in half lengthwise (like they say) before cutting out the quarter circle/corner to make the head hole. Unfold. Head through the hole, belt around the waist to hold tunic in place (or sew up each side enough to make it stay together). And then do the actual toga piece.
--Mimi
Mom to Lala (2004), Bonus Mom to Big Sis 1 (1991) and Big Sis 2 (1992)
Grammy to Big Kindy Kid (2011), Big Pre-K Kid (2012),
Grandbaby Appendage (2014), and New Baby Grandboy (summer 2017)