One of the many collectibles found within Assassin’s Creed Syndicate are the Royal Letters. Finding all 12 of these will lead to unlocking the achievement/trophy, A Life in Letters. They also provide in-depth backstory in the form of database entries.
For the first letter, you’re going to want to head over to this place on the map above in Westminster. It will lead you to a building, where you’ll want to jump the fence and get into the backyard. The letter will be found atop a table, next to the backdoor entrance of the building.
For the next letter, you’ll remain in the Westminster borough and head to this location on the map. You’re going to be looking for a lone bench with stacked books atop it; it will be facing the lake. The letter will found there.
You’ll next come across a beautiful park area with a lake. In the middle of that lake is a small island of trees and shrubs. Within there is another royal letter. Stay around this park, because the next letter is around here, too.
Go to the location on the map, climb the fence, and head up the wall to climb in through one of the windows. The correspondence will be atop a fancy table with a flower and vase on it. Head to another side for this next one.
Head to the location on the map and you’ll see a carriage. Next to it, on the ground, is a suitcase. The letter will be right atop that suitcase, so nab it there.
Just go right through the column’d up entrance and the letter will be to your right on the floor (next to the closed door) ahead.