Easy enough, kill everyone on the starting level and this will unlock.

Baby costume can be found after completing December 23rd, it's in your house in the bathroom which is the blue door next to the staircase.

Screenshot here:

Clown costume can be found in the bathroom of the general store on December 24th, right next to the stalls.

Screenshot here:

Coal miner costume can be found on December 31st, in the warehouse, instead of going into the room to the left with the 4 dancers, go right into the dark area, and another right after that and it's in that square area.

Screenshot here:

To get pyromaniac, the final levels will allow you to use the flamethrower, just complete that using only that and it shall unlock. Beware, you do not get points for flamethrower kills, so you will need to restart the level and get knife kills for S rank.

Pressing space bar will allow you to throw knives. Finish a level only killing with throwing knives. Simple.

I did all these at the same time. Basically to get S rank you need to finish the level quickly, and also score plenty of points. This is probably the most tedious achievement because it takes some speedrunning and time to get used to the character layout, as well as which route to take to score a big combo. I noticed using the left click (short knife stabbing - more points) vs the right click (single hit, only 300 points per kill and takes longer to do this) you rack up more points. On the levels in the houses, this is extremely useful to short click and rack up the combo as compared to stealthily moving about. It will take a few tries, but patience is worth it. There isn't a certain amount of points you need per level, some i got 30000 points in 20 seconds and got S Rank and some i got 9000 points and 20 seconds and got the same. Before you continue to the next level on the final score screen, if you need to restart the level press F2 if you did not get S Rank on that level to restart before continuing.

Here's me getting rank S on every level if you want to try to copy it:

Once you complete the last achievement you need you will get this one as well.