This is extremely easy to do and can be done in the game's prologue called 'Nightcall'.
The best method for doing this is just waiting for your target to head upstairs into the bathroom then follow her. Once she starts brushing her teeth, you'll need to wait until the shower is all steamed up and perform the kill.
You then need to drag her to the cupboard and exit the house through the window in the bathroom, sneak back inside the next door, down the staircase in the panic room where you found the computer USB, throw a coin to distract the guard and make your way to the exit, unlocking this achievement.
Here is a video guide: