Lay your shirt flat on the floor. If your shirt has buttons, we suggest laying it button side up, but after you try out your furoshiki satchel once, you can see which you prefer better. You can always untie it and redo it!
Orient your shirt the way the one in the pic above is with the lower corner facing away from you. Then place your gift in the the center of your shirt so that the edges of it are perpendicular to the corners of your shirt. In other words, the corner of your gift should not be facing the same way as the bottom corners of your shirt (see pic).