In order to unlock the "Snowier Pastures" trophy in Papers, Please, you must escape to Obristan with whomever is left of your family by the final days of the game. It's possible and recommended to aim for this ending during or after aiming for the following endings, as you can reload from the final days of the game to change your path:
You do not need to do anything to prepare for your escape to Obri except to survive until at least day 29. On Day 29, Jorji Costava will tell you that he can help you escape to Obri. You'll need to confiscate enough Obri passports before the end of the game to get you and your living family across the border, plus amass 25 credits per person including yourself.
You must get enough passports to get your whole remaining family away in order to unlock this trophy. You must keep at least one family member alive in order to avoid a game over anyway, so the cheapest way to deal with this ending is to make sure only family member survives to Day 29. It's possible to keep everyone alive (including your optional adopted niece), get six passports and 150 credits before the end of the game if you wish, though there's no trophy-related reason to keep your entire family alive.
Jorji will immediately give you your first Obri passport on day 29 (December 21st) even if you approve his entry. You'll get a citation, as you will for any further confiscated Obri passports.
The sixth entrant on day 29 will be an Obri citizen, as will the second entrant and ninth entrants on day 30. The first and seventh entrants on Day 31, the last playable day, will be Obri. Assuming the game's random generation doesn't cause any of these entrants to have a missing passport, you'll have six, which is the maximum you need. Make sure to keep detaining everyone you can in the meantime to build up some savings should you need more cash.
You can escape to Obristan from Day 29 onwards, including the end of the final day 31, by selecting the option in the end-of-day financial summary. Remember, you cannot get this trophy if you escape without all of your living family members, so be careful.
The trophy will unlock during the slides showing your escape to Obristan.