 Okay you'll notice on the bottom of the ArcGIS screen you'll you'll notice a bar that says drawing. What you want to do is go to the symbol. It may look like a square or it may look like a line. Click on it. Select a line and then just trace the line that you want to put on the screen. When you're done double click. Okay and that places your line on the screen and then to change the to change the elements of that line double click on it and then you can change the color. Here I'll change it to red and I'll change the size of it to two. Hit OK. When you're done with the line the way you want it click outside and there you have it.