You get this achievement for successfully making oatmeal and getting back into your room with it. The only major trick here is to
1. At the beginning, after getting the oatmeal and spoon and mug, but before leaving your room (basically back where the save point brings you if you fail), do not immediately leave the room (choose "No, uhh...." ), then choose "What about the things?" This allows you to remember to take your keys with you (Note: failing to take your keys is essential for another achievement).
2. Make the oatmeal. See
3. After you make the oatmeal, be sure you use your shirt to pick up the mug.
Beyond those, you should have no trouble getting back into your room and getting the achievement for completing the (very very short) game.