Ride on Car controller

I have written some code in Arduino IDE for the Arduino Nano. I have 3 inputs and 4 outputs. The code I have written works fine, however, I need the motors to ramp up to the desired speed set by the potentiometer when the pedal is pressed and ramp down to a stop when the pedal is released. The ramp time of the motors needs to be a variable I can adjust.

All other functions I have in the code need to work as they currently do but with the ramp up/down ability for the motors.