Pretty straightforward collectible achievement here. One thing that's cool about these though is that progress into the charlie dolls will persist across all playthroughs, regardless of any extraneous factors like dying after you get one and reloading to before you got it. Even in that scenario you still have the doll.
Two videos below. One is my charlie dolls video, and the one below it is my video for every achievement in the game, which includes charlie dolls of course. Both have time stamps with them. Take your pick and I hope they help
Charlie 1 - 0:00
Charlie 2 - 0:30
Charlie 3 - 0:52
Charlie 4 - 1:11
Charlie 5 - 1:26
Charlie 6 - 1:43
Charlie 7 - 1:59
Charlie 8 - 2:17
Charlie 9 - 2:47
Charlie 10 - 3:08
Charlie 11 - 3:28
Charlie 12 - 3:44
Charlie 13 - 4:01
Charlie 14 - 4:23
Charlie 15 - 4:40
Charlie 16 - 5:02
Charlie 17 - 5:25
Charlie 18 - 5:44
Charlie 19 - 6:04
Charlie 20 - 6:16Charlie 1 - 3:13
Charlie 2 - 4:58
Charlie 3 - 6:45
Charlie 4 - 7:46
Charlie 5 - 10:29
Charlie 6 - 13:48
Charlie 7 - 22:05
Charlie 8 - 23:57
Charlie 9 - 26:04
Charlie 10 - 27:57
Charlie 11 - 30:51
Charlie 12 - 32:55
Charlie 13 - 33:12
Charlie 14 - 34:27
Charlie 15 - 34:56
Charlie 16 - 37:26
Charlie 17 - 42:15
Charlie 18 - 46:53
Charlie 19 - 49:50
Charlie 20 - 51:11