Edit 3: *Seems like the latest patch has finally fixed the achievement.*
There are 47 paste-ups and can be obtained any time. For it to be easier use the construction worker which you get in an early story mission and use the cargo drone to get to the location easier.
Be sure to use the same character when going for them and I think this is the most important step. Use the same image for every paste-up! I'm not sure this makes the achievement 100% pop but this is what worked for me when it didn't unlock the first time I went for it. Edit 1: Confirmed by Wookiee Hairem that it doesn't need to be the same image. Edit 2: It looks like the Ubisoft servers are messing the achievement's progression because of the servers disconnecting very often.
You can use x ii Y1D x's suggestion from the comments below: Go and find out which ones you DO have using the video guide, and note down which ones you DON'T have. When going through all of the paste you haven't done, do about 5-10 of them. Quit the game and load back in and check the last one you completed, and if it's complete then you know the saving is working. So I just did some at a time until I finished the last one, and bam it popped!
JuIIIian provided a map in case you want to use the map rather than the video: https://mapgenie.io/watch-dogs-legion/maps/london
For the locations this video showcases very well every single one:
(It's my first guide so any criticism is well appreciated)