So, what does Don’t start with a character waking up! really mean?

Don’t be boring.

Agreed. I mean, Kafka’s rather famous work “The Metamorphosis” literally begins with a man who wakes up as a giant insect. And that’s certainly not boring.

I like how you looked beyond the literal rule or guideline (“don’t start with a character waking up”) into the reason for its existence (in this case, it’s “don’t be boring”).