1. Your sim must be as fat as you can make them in Create-a-sim.
2. If you have two sims then both sims must participate
3. Your sim(s) must have a tech guru job, and can not have the athlete job.
4. Your sim(s) must have the lazy trait, and they can not have the athletic trait.
5. If you have two sims at least one of them has to have a job.
6. If you want your sim to get pregnant they must exercise even while pregnant.
7. Your sim has to exercise at least once everyday and they must keep doing it for the entire time.
8. You must have a tredmill and iron pump. You can't have a punching bag.
9. Your sim can't get their fitness skill up other than exercising.
The Goal: Get your sim to be skinny or have their fitness skill to level ten.
I hope you try and enjoy this challenge. If you search my Origin ID(thesims3mermaid) on the TS4 gallery you might see a family that you can use. It is called the Ugly challenge. If you can't find it I will be sharing them soon. Enjoy!
Or do both and create yourself as a mermaid.