I just did a test to figure this out. Writer's block appearing is random, but seems to appear when you've written for long stretches. When I wrote only 1.5 Children's books at level 1, I got the Writer's Block Moodlet. That was too easy and didn't teach me much of anything.
I wanted to see what it would be like for a level 10 writer, so I reloaded and cheated my Sim straight to level 10 writing. She wrote 3 books then practiced writing for a bit and got the Moodlet. It did appear with Practice Writing. I didn't experience this Moodlet at any point at this level before, so think that only by doing a LOT of writing at high level will it pop up. Maybe it won't even come up when you're level 10 if you've already had it before. I didn't use testingcheats, so it was something doable for any player. I would try to get as much Writing in as possible within a 24h period to make this happen if you're already higher level, and perhaps use Practice Writing to minimize downtime and force them to stress a bit.
I'm doing another test to see if I can get something consistent - like how many hours of practice writing it typically takes. I'll edit in my results to try and help you.
Edit: OK. Test 2 confirmed something for the Guide. Practice writing is the way to go. I had two Sims, one level 10 the other 5. I figured I'd have the 10 do Practice Writing while the 5 did Children's Books. The level 10 Sim got Writer's Block after 5 hours of Practice Writing 2x in a row, while the other Sim was on his 4th Children's Book and about 14 hours in. Practice Writing is far more stressful, as it doesn't stop if the Sim's needs are up to the task. Go Practice Writing as long as you can and let us know the results!
