To unlock the Clark Cable trophy in Cities: Skylines, you will need to purchase and install the Mass Transit DLC.
For this trophy, you will first need to unlock cable cars once you reach the Small City milestone at 2,600 - 11,000 population depending on which map you start on.
Once you have cable cars available, you ideally want to place stations at your busiest areas, large industry zones, commercial areas, transport hubs etc to get the maximum number of passengers travelling as possible. The more stops you add in busy areas, the better.
Once you built a decent route, carry on playing the game as normal or fast forward on 3x speed until the trophy unlocks. When it does unlock, carry on going, as there is a second trophy that unlocks once 20,000 passengers have used the cable cars.