Each of my virtual students should have a piece of colored mat board and a couple different colors of yarn. You'll need to make notches along the edges, about 1/4" deep. If you cut 12 notches on one side, then you'll need to cut twelve notches on the side that touches it at a right angle. In fact, let's keep it simple and make the same number of notches on all four sides. If you have a rectangular board, the space between each cut should be longer than those on the short side. Do the math.
You'll stretch the yarn to the notch second from the top on the left side and then bring up the notch directly under it. Continue the pattern, by pulling it down to the 3rd notch from the left up the fourth, and then back to the next notch on the left side.
If you are right handed you may want to start your project on the upper right notch and go to the bottom, closest to the right, but the idea is the same: move over one and down one, until you've used every notch.
When you finish using all the notches on those two sides, you'll notice that all of your straight lines made net that makes a curve along the edge.
Try doing the same thing with the same color yarn, with the opposite corner by rotating the board 180 degrees to bring the top to the bottom and the left to the right, keeping the colored side of the mat up.
This ends up with an diagonal football shape. You may want to choose a different color yarn and do the other two corners, to create a balanced finished piece. This can be a frame by tucking corners of a photo or drawing behind the string, or it works as a stand-alone work of art. Either way, you've managed to do some magic by making curves from straight lines.