First, you need to have picked up a camera during one of the mission briefings (Act 1 through Act 4) using the Mark II/III. The camera can be found behind Sunny's desk during Act One, Two or Three's mission briefing, or in the kitchen during Act Four's Mission Briefing.
There are 30 ghosts on Shadow Moses (Act 4), but you only need to take pictures of 5 of them. You can get the 5 needed for the trophy at the heliport.
You won't be able to see the ghosts until you actually take the picture (unless you have the sorrow doll equipped). So you have to point your camera at a certain spot, and when you snap the picture you'll see a ghost (they are still hard to see though).
This video shows you how to get 5 at the heliport quickly. Credit to kamijordan on youtube for making the video.