Okay, I FINALLY did it.
1. Have any sim get good at painting.
2. Get that sim angry by performing mean actions.
3. Click on the easel and paint "angry painting."
4. Make many angry paintings.
5. Create a separate room just to put angry paintings on the walls.
6. Put five easels in that room and fill them with angry paintings also.
7. "Enable Emotional Aura" on all angry paintings, if you haven't already. (I literally have 11 angry paintings enabled in that room.)
8. Place a desk, computer, and chair in that room. **Make sure you don't use one of the chairs that inspires an emotion.
9. Have your writer sim walk into the room and "View Angry Painting."
*This is important. Just being in the room isn't enough. Viewing the painting will make the sim "very angry."
10. Make the writer sim write for hours. Keep giving him or her new things to write if they finish others.
= After about 15 hours of very angry writing, my sim achieved "Beat Writer's Block" and moved on to step 2 of the Best Selling Author Aspiration.
A lot of work just to get to step 2!