Bob,
You can use a flexible ruler. Just add a sufficient amount of masking tape to the bottom to raise the edge above the surface to prevent the ink from wicking under the ruler.
The great thing about adding panel lines is that you can easily erase any mistakes with a paper towel and alcohol. Just be patient. You are going to smear a line or two. Just erase it with the alcohol and start again. When you are satisfied with it, you can hit it with the clear to make it permanent. Most importantly, have fun!
Steve.