Ok, so since the other guide is less than in depth, I thought I'd write something a little more encompassing.
In order to earn this achievement, you need to wait until Chapter 5 of the game. You will gain access to the last area of London, Temple Church (part of the East End). In the actual church, or rather under it, you will meet Usher Talltree who will give you a Citizen Mission that involves going back to the Theatre (where you will have been as part of the main story) and collecting his journal from a safe in the basement.
Now, annoyingly, you can only return to the theatre as another part of the main story (
). But when you do you will need to interact with the safe (Jonathan will comment on needing to find the key or something) in the basement. After you interact with it, go back upstairs and new enemies will have spawned that will drop the key when killed. Once you collect the journal DO NOT READ IT
, return it to Usher and he will give you one of the collectables (The Brotherhood of St Paul Stole - Origin of the Brotherhood). He won't give you this if you read his journal.
On this collectable will be a little diagram that refers to the little hallway near Usher and the very obvious plates in the floor. Now for the tricky part, you need to look through pretty much every other collectable in the game to find ones that have dice-like spots on them and a symbol. Using the dice spots as the number in the order and the first collectable diagram as which plate corresponds to which symbol, you can work out that you need to step on the plates in a specific order. There will be a total of 6 in the order.
The order can be different from save to save, mine was the same as in the other guide on my first playthrough but completely different on my second. Actually doing the order seems to be a little finicky/glitchy, as it took me more than one attempt but seemed to work best if I did the order as quickly as possible.
Oh it is possible to glitch through the wall by attacking it but this will only work if the region is Hostile (otherwise you can't just randomly attack the wall) though you also apparently won't get the achievement, just access to the weapon. This is obviously useless to get this achievement, but helps if you just want the weapon as part of the achievement to collect all 1h weapons.