Create an invisible sprite called "Camera". Apply the Scroll On behavior to it and using events move the Camera sprite a little bit every tick. You can try to apply the Scroll On behavior to the character sprite if you want the character to travel more to the left and right, but it is unrecommended, because the Rule #5 of Construct is:
THOU SHALL NEVER APPLY A SCROLL ON BEHAVIOR TO 2 OBJECTS AT ONCE
You can try, though, I think it will work fine.
HARMLESS SIGNATURE THAT IS VIEWED BY OTHERS AS OFFENSIVE AND CONTROVERSIAL