The first time you will come across snot bubbles is in Act III, Chapter II: Night Terrors. After
, you'll move on through different buildings and tight streets dealing with Juvies.
Eventually you'll enter a house and while going up the stairs to the second floor, Kait will notice the snot bubble and you'll be able to look at it as a POI by holding down
.
Shoot the bubble until the Mulcher stored inside drops down. Turn to your right and look up to the ceiling to find the second one above the stairway you just went up. Reload the checkpoint and repeat.