50 Stealth kills on human NPCs is fairly easy and will come naturally doing the other stealth-based perks.
However getting 5 stealth kills on the smaller dogs is harder, and you won't have many opportunities early in the game. To finish this perk quickly, farm these kills in Chapter 1. Once you find the 1946 handgun silencer, silently headhsot these dogs as you find them in the chapter. A good spot is the patrolling dog that sits down in front of the commanders room later in the chapter, or after you go up the circular stone staircase before you reach the room with the large painting and suits of armor. Either way, I recommend farming all 5 of these silent kills in Chapter 1, as it'll be a while before you find these dogs again, and opportunities to stealth kill them aren't numerous.