Step 1 - Main Campaign
Your first step on your journey to the Platinum Trophy should be to play through the game normally. There's a lot of story-related Trophies to unlock and along the way, try to complete as many Side-Missions as you can, some have Trophies associated with them. Note that all of the story-related Trophies must be completed with you as the Host or you won't get the credit. Try to have a friend join you for at least three missions to unlock the related Trophy. During your main playthrough, try to conserve your money as much as possible, this will make it easier to unlock a Trophy down the line.
Step 2 - Campaign Cleanup
Once the story is complete and all islands liberated, work on finishing up any Side Missions you may have missed. Remember to speak to everyone that you come across to unlock the associated Trophy (talk to 50 Citizens) and spend $50000 on Vehicles and $1000 on Clothes too. A lot of the Trophies in this section will be the misc related ones and the game is kind enough to not require you to 100% everything. For the collectibles, only finding one of each is enough for the Trophy as is the Cult Properties you find throughout Hope County. By the end of this step, you should have unlocked all of the Campaign Trophies.
Step 3 - Arcade Mode
Arcade Mode is Far Cry 5's multiplayer mode and there are a few final Trophies to wrap up here. Firstly, you'll need to reach Level 20, something that shouldn't take too long if you've played a lot of Far Cry 5 already. Play the Hero mode as you do this for the additional XP and to unlock the Trophy pertaining to that mode. The most difficult Trophies here are winning on the featured maps, unless you're boosting with a friend it may take a while to find the Ubisoft created maps amongst all of the player made ones.