Last year, I realized that the ONC was really a contest to see who could write the best first draft. With only 12 weeks to write, it's probably pretty difficult to not only write 20,000-40,000 words, but developmentally revise then line edit them. Unless, of course, you're a writer unicorn.
Unfortunately, there is nothing magical about me and my writing proccess. That's why I rely on my outline during the ONC. I recommend trying to plan the three major story elements: plot, characters, and setting.
For outlining the plot, I'm somewhere in the middle of the outliner/adventurer divide, so I like to assign my chapters story beats and then write a short summary. If you're not a big planner, at least try to have a road map of your story's plot whether it be bullet point lists or a sentence for what you plan to write in each chapter.
Here's an example of what one part of my outline looked like for "NOVYYE SAINTS" last year.
It's simple paragraph, but it gives me an idea of where the story needs to go in the first sentence. It also describes what the chapter should accomplish in terms of story and character development. Word counts are also critical for the ONC milestones, so try to identify where you want them to fall in your novella.
For character outlines, I usually just jot down characters sketches instead of meeting them as they develop on the page or filling out an extensive character questionnaire. Key things to note about your characters include their desires, motives, needs, deep flaw, backstory, role in the story world, and any specific/noteworthy details.
As for setting, planning for this probably varies the most. Some writers create whole fantasy or science fiction worlds for the ONC. Others stick to the present. I write historical fiction, so my work is a blend of these two variations. I did create a fictional city for "NOVYYE SAINTS" and also set "THE DEVIL ANGELOVSK" in the same city sixty years later, so I was sure to draw a map and write some details about the city districts and some notes on how they're different/how the city has changed over the decades.
Overall, having these three elements planned out before you start the ONC is critical. Not only will it help you combat writer's block, but it'll ensure your story is coherent, cohesive, and has an arc the judges will love.