You've got a neat idea you want to turn into a finished thing (animation, VN, action game, whatever).
There are a few general steps you can take to help you get it done on time:
1. Define what your end product is- be specific and write everything down
2. Plan out how to make it- be specific
3. Check with other people to make sure your plan and product look realistic
4. If it looks like you won't get done in time, look at what you can take out, or if you can add new people to help
5. DO NOT ADD NEW FEATURES
The hard part about a project is getting it done. One thing that will help is to make sure you know the scope of a project when you start it. If you stick to the defined scope, you'll avoid a lot of problems and stand a better chance of finishing your project.
Okay, but what's scope?
- Project Scope: "The work that needs to be accomplished to deliver a product, service, or result with the specified features and functions."
- Product Scope: "The features and functions that characterize a product, service, or result."
That's a bit of a scary definition, especially if you're just doing something simple like a little game or animation, right? Let's use some examples to make it easier to understand. We'll start with product scope.
"This is a 2d platformer, it has 3 levels of varying difficulty. You can choose between two characters, and they have three identical and two different moves. There are five different enemies and two bosses. The game will be playable on PCs and Android equipped devices..." and so on.
"The animation is two minutes in length, and will have music but no voice acting. Subtitles will be used instead. There are five scene changes and each scene change will have a number of cuts, to be described below." and so on and so forth.
Project scope flows pretty naturally from the product scope. Once you know what you need to do, you can pretty much figure out how you'll do it. You should be pretty specific when defining scope. Taking the time to write out a specific document now will save you headaches later.
Once you have defined the project's and product's scope, check with other members of your team, and outsiders, to see if it sounds realistic given the resources (money, equipment, skills, etc) you have, and the time you think it will take.
If you want any chance of succeeding, add nothing to the product scope. Don't add a new scene, or a new path, or a new enemy or a new story, or voice acting or anything. Add nothing. Maybe that can go in the sequel or a new project.
Adding stuff after you've defined the scope is called "scope creep" or "feature creep". You will kill your project, and waste all the effort you put in to all the other parts.
That's basically it. Plans can be modified and changed, but mainly that means subtracting. Adding stuff once you've started the project is bad news.