Some tips for writing a humorous narrative is that you have to start with an idea. I think that if you don't already have an idea or topic in mind, then you will definitely struggle when you try to write a humorous narrative. Another tip would be to make sure that your narrative is appropriate for your audience. You really don't want to offend anyone, you also wouldn't want to make a joke about something that the audience doesn't understand. You should also test your jokes on someone that you know before telling it to an audience. You might think the joke is funny, but everyone else might not.
So my comedic monologue is about my old brother Spencer. I originally got this idea when I was trying to think of a joke that we had to do one night for homework. I was doing the dishes and I saw my brother watching YouTube videos. The video was of people trying to do cool things but they ended up messing up. My brother calls them "fail" videos. So then I instantly thought of my first joke. It was about how my brother likes to watch "fail" videos on YouTube but he doesn't laugh when I hit him in the face with a soccer ball. After that I decided to make my entire monologue about my brother. It was mostly about the things he does and how I relate to it. I think the audience will definitely laugh because a lot of people don't get along with their siblings and my monologue really talks about that.