SmashX1. Easiest way to start is by putting all your ideas out on paper, personally I find videos I think are funny and start imagining how I could animate it.
2. Just draw? I’m not really sure what this question means.
3. You could draw the thumbnail and storyboard it in your notebook and then use the DSi camera to take a picture and import it into Flipnote Studio to clean up.
4. (refer to my first answer) But if it is isn’t anything I’ve watched that I thought was funny, I usually get ideas from talking with my friends and/or ideas I’ve gotten while doing my daily tasks.
Hope this answers your questions and helps!