How do you become a fashion designer on sims freeplay?

1. Build the Promotions R Us store.

2. Go there and buy the Fashion Studio. After you pay for it, it'll appear in your Inventory.

3. Take it out Inventory and into one of your houses.

4. Click on it to have a Sim start working there.

That's it. You'll start as a Fashion Disaster and move up with experience.