There are two scenarios that possibly got you here.

1. You just started playing Returnal and want to prepare for the Achievements.

2. You have played already Returnal and are interested in how the Ciphers work, or where to get those you are missing.

For the former ones, you are perfectly fine! Preperation is key for the easiest time, gathering the ciphers.
For the latter ones, well, you will most likely suffer a bit more than the other ones. But i will keep explaining why in the following guide.

So there are a scripted amount of ciphers in each biome of the game. If you want to stay spoiler free, beware of scrolling to far down the guide, since it gives away which biomes exist.

Each biome has a set of rooms which have different possibilities of spawning in a playthrough. The ciphers are hard written in those rooms. If you have found a cipher in a specific room, there won't be another one in this specific room. That is why the people preparing for the collection, do have an easier time. You just can keep track of the rooms in which you found a cipher and can "ignore" these in future runs. If you manage to remember each room that you collected a cipher in, this is a big win for you.

In the following, i will give you links to videos that already exist on youtube, since they helped me a lot for finding all of the ciphers.

I do not own any rights for these videos.
I am just embedding them on steam for easier acess.
All rights reserve to VideogamersEU

Please make sure to read the video description too, to get a full understanding on the classification of rooms, made by VideogamersEU.