There's a lot of maps and videos floating around on the internet for this trophy, but frankly you shouldn't need any of that. All you need to know is this:
Every time you fly underneath a bridge, you'll see trophy icons appearing on your mini-map. Press start to see the whole map, with trophy icons at the bridges you didn't fly under yet. Just place a marker on the next bridge, fly under it, and repeat.
You can get to a helicopter as soon as you get to the central island (Algonquin). There's a helicopter tour business on the south side. Also, once you get to the big island, if you have done all of Brucie's missions, shortly after getting to Algonquin he'll phone you to ask you on a helicopter ride, you can just do it during that 'mission'. Once he's your friend you can also call him yourself and select 'heli'.
Another tip: Do this while you can't yet officially get to the last island. That way, when you get to those bridges, you'll get a six star wanted level and you can go for the One Man Army trophy. Staying alive for six minutes while you're high up in the air is a piece of cake! Doing this during Brucie's activity has the added benefit of the wanted stars disappearing if you get back to his heli pad afterwards.
Last tip: There are four bridges on the left island; the most left of which shouldn't be flown under while you're over the water, but slightly south of this, over land. The bridge won't count as being flown under if you're going over the water.