The planning of a film goes through various stages to lets take them one at a time.
The first thing you need is a story, you can write your own, or adapt an existing one. Animation tends to be a solitary thing, don’t lock yourself away and keep everything a secret. Show your story around, discuss it, listen to other peoples ideas, you may be able to use them, or reshape them. When you have a story you are happy with, put it away in a drawer for a couple of weeks, when you reread it, if it still sounds good then you are ready to start.
The next stage is to design your characters, again don’t be a hermit, remember people are going to see your final film, so get their opinions. You may think a character fits a role, but an audience may not, so listen to their comments. Once you have designed your characters, the next thing is to draw a model sheet for each one, and a model sheet for their relative sizes to each other. A model sheet will help to keep your characters consistent. When you design a character keep it simple, for ease of animation, a complicated character is hard to keep consistent.
You now have a story, and the characters to act it out. It is now time to draw out your story board. This is the time for chopping and changing. What. sounded good on paper, might not look good visually, or you might come up with a better idea for a scene, or find a scene does not work, now is the time to sort it out, as the story board is what you will, follow throughout your production.
Again, show your story board to people and ask them to explain it to you. If they can, then you have a successful board, if they get confused, then go back and rework it, an unclear story board will result in a film that is hard to follow, it will, also not hold an audience’s interest.