As the trophy description states, you must finish the game along side the same partner through the majority of your journey.
If starting from the beginning that means you need to maintain the same companion starting from the Bridge chapter all the way through to the final chapter, Sky.
A quicker and easier way to do this though is to first complete the game. Once you do you will unlock chapter select which allows you to pick the starting point of your journey (making your overall journey shorter and the length of time required to have the same partner shorter as well).
Select the second to last chapter, Snow. Wait at the beginning of the level until you meet a companion and then complete the game.
Once you walk through the white light together at the end, either using the first method or the second, and watching the end credits the trophy should pop.
Also, if you haven't received it already, this should also pop the Crossing trophy. Though it is not confirmed, as I received them at different times.
**Note** This trophy can be known to glitch if at any time you or your partner are syncing your trophies before the end of the credits. To be safe I would just wait until the trophy pops before I checked/synced my trophies.
Hopefully this helps and have fun!