To successfully make oatmeal you need to do a few things:
1. Leave your room and head to the kitchen
2. Once in the kitchen, wait for the water to heat up
3. After the water is heated and you've tested it, use the microwave
4. Choose to stir the oatmeal rather than let it overflow
5. Keep cooking it, don't just decide it's good enough
After all of those choices you should get the achievement.