Okay, I know that this sounds crazy, but it isn't actually too difficult. I know how you feel, I spent ages going through comment after comment through forums in hopes of finding an answer—eventually, I just tried my own thing:
- So the first thing I did to achieve writer's block was to create a small, plain room outside with the room tool. I placed a desk and a computer in the room and got rid of any decorations. (Make sure that your sim is not around any inspiring, happy, confident, romantic or energised decor)
- Next, I made sure my sim's essential needs were up, bladder and energy especially. It's okay if hunger, hygiene, fun or social needs are moderate and/or even lower—this has a chance of making your sim even more miserable.
- Finally, I sat my sim down in the boring room and "Practiced Writing" for a while. As her hunger went down, as well as her fun and social needs, she became more and more uncomfortable.
- It took me about 7 sim hours, but my sim finally reached Writer's Block! Now all you do is perk your sim back up again in the four hours of the stressing moodlet. Read a book, listen to music, feed them, take a bubble bath, anything, until you finally beat Writers Block.
(Tip 1: The most important thing to do is to be patient. I may have been very lucky, it might take a lot of triple fast forwarding, just sit back and wait. Before starting the hours, clear any interruptions (ie: work, social interaction) delete the door if you must.
Tip 2: Don't stress if the hunger need is in the red zone. You'll see that your sim has 24 hours before they become "ravenous," and that's plenty of time! Keep on fast forwarding until the mood let says about "4 hours until ravenous," that might be a good idea to start feeding them.
Oh and PS: My sim on level 10 writing when I got writer's block. But I don't think that the skill matters.
Good luck! Remember; be perseverant. Make those hours hell for your sim.