Smashes are the yellow fences in front of shortcuts.
If the fence looks intact, you haven't hit it yet, so stop what you're doing and drive through. Once you've got it, it will always be clearly broken in the future.
There are shortcuts all over the city, so you really don't need a guide for this one: get driving!
If you'd like a checklist, though, check out the Burnout Wiki (click 'Show'):
https://burnout.fandom.com/wiki/Smash_Gate#Smash_Gate_Walkth...Which also has a map:
https://vignette.wikia.nocookie.net/burnoutparadise/images/f...All 400 smashes will also get you