How do I complete the den clothing goal?

I think you mean the design clothing goal. First you have to build Promotions r us (I'm assuming you haven't already.) You have to buy the promotions r us store from The Town Map. It is beside the Pet store, which is beside The Salon. Then go to the Town Map and click on it once it is built and you will be able to buy a Fashion studio, which costs 2500$. It is like a piece of furniture, just go to your inventory after buying it, you will find it under the Hobby item tab. Place it in your sim's house. Click on it and your sim will have the option of using it for different time periods, I think the shortest one may be 1 hour. Once the hour is over you click on the exclamation bubble above the sim's head and then click on it again when a lightbulb appears, don't click if it's a bomb. This might not make much sense right now but it hopefully will then. Anyway once all that is over you should have completed the goal.