Be aware! This achievement is glitchy. I had to reload an earlier save and make the hot chocolate again to get the achievement.
Edit: I would suggest completing the first chapter before starting on this achievement. There are reports that you are unable to complete it until afterwards.
We are gonna make some hot chocolate! Let's get started! Here's where the ingredients are at:
Hot chocolate packet - In the bottom drawer next to the oven.
Milk - Inside the fridge. Back on the middle shelf.
Now that we have our ingredients let's put them together. Go to the tray next to the microwave and place your milk and hot cocoa packet next to the lonely cup. You will get a cutscene where you place the hot chocolate in the microwave. Next, your hot chocolate will dissappear and go into another dimension. Head up to the boy's bedroom upstairs and you will see a swirling vortex at the end of the room. Walk up to it and a cutscene will play with you enjoying some hot cocoa.