Step 1 - Working on Missable Trophies During Main Story
Your first step in obtaining the platinum trophy is going through the main story, while getting the missable trophies. It should go without saying that working on the Gold Medals and 100% during this step will make things a lot easier after completing the story. There are five potential missable trophies in the game, listed below, with three of them related to your gang's camp.
-- Breaking and Entering -- Errand Boy -- Friends with Benefits -- Give to the Poor -- Lending a Hand
Errand Boy, Friends With Benefits, and Give to the Poor all involve your gang's camp, so those must be done prior to finishing Chapter 4. Breaking and Entering can be a little buggy, but it's best to do this during Chapters 2 or 3, just in case. Lending a Hand, however, encompasses six chapters and requires finishing all optional Honor Missions. Basically, any mission that isn't marked with a question mark and doesn't advance the plot is considered one of these missions for Lending a Hand. The biggest problem is that your Honor needs to be high enough during Chapter 6, as there is a missable mission then.
Step 2 - Complete Main Story
Once you have gotten the above trophies out of the way, go ahead and finish the main story, including the Epilogue missions. The above trophies will carry you through to Chapter 6, with there being two sets of Epilogue missions after that.
Step 3 - 100% the Single Player
This step will have you completing the 100% Checklist that you see in the Progress option of your pause menu. It isn't a true 100%, as you don't need to do literally everything in the game to achieve it. You will receive the Best in the West trophy for doing this and while most things are pretty standard, one of the more time-consuming aspects of the game will involve completing all nine challenge sets. The collectibles will also take a lot of time, although not every single one is needed. The compendium section of the 100% will mostly be finished as you do other trophies.
Step 4 - Work on Gold Medals
While you should try to get some Gold Medals on missions during your first play through of them, this step will concentrate on them. Note that you won't be able to have access to your entire inventory when replaying missions, as the game uses its own set tools and weapons for them. This can make some missions harder to get a Gold Medal on than during the actual game. Luckily, you only need to get 70 of them, so you don't need to do all missions in the replay list.
Step 5 - Cleaning Up the Single Player
You've done all of the above, but there are some other trophies that don't fall into any of the other steps above. You will use this step to earn those trophies, with a few tied to side missions, although most are completely unrelated to each other.
Step 6 - Red Dead Online
The current multiplayer is still in beta, but you can earn the online trophies while playing right now. The worst trophy of the bunch is definitely getting to Rank 50, but you should be able to get everything else before then.