The Hauntings in the Garden anthology is an excellent way to study the variations in length, story style and genre you can fit into a short novella.
The next step is to plan your own.
Outline Your Novella
An Internet search will produce hundreds of documents, all aimed at helping you outline a novel. I spent years following the
Snowflake Method. It is thorough, effective, and works, but in order to produce a novella in a limited time, I felt the need of a simpler step-by-step plan.
Searching for this article I found the helpful resources page from annie neugebauer and liked the pared-down novel help sheets as they outline the steps to take one by one.
I have never been able to master the ups and down of story arcs, the graphs, the long explanations. I just want to get on with the writing but I needed a flexible plan to keep me on track for the days I get stuck. No, it's not writer's block. I just write myself into a hole that I can't dig my way out of :-)