Put the two selvages together. Lay the fold line against one of the lines on the cutting mat. Then lay the ruler so the lines are matched up and cut. Just use the cutting mat lines as a reference point so you are cutting straight. Good enough to go!
Some tips!
https://www.youtube.com/watch?v=dVqqefuxa2w
https://www.youtube.com/watch?v=EcpzwJMVTbc
Welcome to the boards!