To start a Showtime you need to press and together when not in an event.
You gain a 1x multiplier to your counter for each Bus
you hit into so you will need to hit into 4 Buses to get this trophy, I suggest you pick a busy road like 1st Street and only enter Showtime when a Bus is in your sights, then try to keep going to find more buses.
A 5x multiplier isn't too much to ask.
Note getting this trophy will also unlock