The best way to plan a novella is using a mind map or a ‘brain-dump’. They always work and don't take much time, either.
You did these in school - the large, squiggly diagrams that look obscure to the unknowing onlooker. Mind maps have been successfully proven as useful tools within university, office and creative settings.
When mind-mapping for your novella, try coming up with three separate ‘branches’. The first branch could be what you’d like to talk about and why; what does your novella want to confront, are there any messages you’d like to portray and what novella form does it fit into?
The second branch could address your scenes or sections; how many parts do you want there to be to your novella, do you want your story to be linear or to time-hop, and how will you go about chunking this?
Your third branch could be extra details, such as character name ideas and traits, settings, languages, and so on and so forth.
While mind-mapping does not write the novella for you, it does give you a solid structure. Mind-maps also remove some of the fear of writing. You can see the ideas in front of you starting to link together, the thought of reaching that particular word-count doesn’t seem so stressful.
Importantly, mind-maps give you something to refer back to; at any point you can come back to your branches, cross off parts you’ve written and chart your progress.
And mind maps are also wonderful in terms of additions. If you suddenly have a fantastic idea for your novella, simply extend your branch and work it into your plan.