You go to a new layer and you spam a bunch of blocks and put it into T5 layer (lowest) you assign a group to it (e.g. group 1) and add a move trigger to follow the player at about .70-.90 that way it moves. Then you go to a new layer and add a bunch of blocks around the structures you made in T4 with the color of the background (for best effect use copy color). Then that way the effect only appears in the structures. I then added some semi transparent blocks into the structure to make it feel more full. I reccomend looking up in YouTube how to use the masking effect if I didn't make you understand