Step 1: Complete the game on any difficulty, find all collectibles
Your first objective in obtaining the Platinum Trophy is going through the game on any difficulty whilst thoroughly searching your surroundings for Artefacts, Trading Cards, and Coins. During your playthrough you'll also find mini competitions to participate in, such as Target Practice with Manny during Seattle Day 1 - Abby - The Stadium. Refer to this walkthrough to help find and do all that Seattle has to offer.
Step 2: Replay the game to acquire the remaining skills
Once you've beaten the game once you'll need replay it on any difficulty, as it is impossible to find enough scrap/supplements in the world to fully upgrade your character in one playthrough.
Every Last One of Them
Collect all trophies