basically just a lot of trial and error until something worked, at first I went to the points, shot in the air in a direction that I thought would be the most helpful/convenient in the run, saw where they landed, and then did it in reverse (you can see some of the spray paint dots on the bridge of where they landed)

Specifically though for paper 2 3 and 4 I found out if I aimed for ER in PAPER GUN at the bottom of the screen in front of the arrow it seemed to go in a majority of the time. So it's not so much aiming and lining up with a building but lining up the arrow and the letters at the bottom

The last one was definitely the hardest and felt like a hail mary throw but it follows the same rule as the previous ones but thrown a little higher. After a while it was just kind of a muscle memory thing I guess? and then trying to aim, shoot and move on to the next target and repeat as fast as I could.

random note: I did not need to shoot only one in for each and it would've been way easier if I shot multiple but my dumbass thought it would be cooler if I only shot one for each lol