How do I download and save a You Tube video in my computer?

First is download the Internet Download Manager.This is the one I' am using.
After downloading, install the IDM in your computer or laptops
And then, if the IDM is already install. Open www.youtube.com and choose a video you want to download.
After clicking the video there's a box that will appear.Click the box and click start download.
After downloading the video,convert it into mp3 avi or any format do you want to watch the video.
Thats all.