How to convert .class to .jar

I have searched and visited many sites that teaches how to convert .class files to .jar but none of their methods work.

But this site's method works after searching for some time. It is http://www.roseindia.net/answers/viewqa/Java-Beginners/9375-I-want-simple-steps-to-create-Jar-File-of-Java-Project.html

The site above teaches you how to convert .class files into .jar files using Command Prompt.If you want to convert them using programs, you may try searching on download.com