Step: 1 - Play through the story & conquer the combat Trophies
Note: There isn't any missable Trophies/Achievements within Star Wars Jedi: Fallen Order.
Your first step towards obtaining this Platinum Trophy or maximum Gamerscore is to just play through the game on any difficulty that you desire. Use this walkthrough to help find and do all that the galaxy has to offer, make sure to scan every new enemy that you defeat and get BD-1 to scan everything you come across as there are Terrarium Seeds to find which will make Greez happy and it will knock off a Trophy/Achievement.
During your play through try to kill enemies using different methods, such as killing them with their own explosives, performing perfect evades, and pushing them off the side, as these will work towards serveral Trophies/Achievements.
Step 2: Obtain the remaining collectibles & complete the rest of BD-1’s holomap
After the story has been completed you'll be able to revisit the previous planets and collect any remaining Secrets and Chests that you may of missed or didn't have the ability for at the time, refer to the Force Essences, Life Essences, and Stim Canisters pages to locate all the Secrets in the game, then take a look at the Ponchos, BD-1 Skins, and finally Mantis Paint Job pages for all the Chests.
Upon getting all the items that the game has to offer you'll now need to look at all the zones within the planets and make sure each one says 100% to fully complete that planet.
Step 3: Defeat any Legendary Beasts & Unlock all Jedi Skills
Once you've beaten the story and found all the remaining collectibles you'll need to take out the four Legendary Beasts, Oggdo Bogdo which can be found in the Bogdo Sinkholes in Bogano, Rabid Jotaz which can be found in the Broken Wing of Zeffo, Albino Wyyyschokk which can be found in the Imperial Refinery of Kashyyyk, and Nydak Alpha that can be found in the Nightmare Ruins of Dathomir. Once they've been defeated you should have enough Skill Points to purchase the remaining skills, if not just farm some Stormtroopers until you do.
After everything above has been done you'll be a proud owner of the Star Wars Jedi Fallen Order Platinum Trophy.