Please explain what you didn't understand.
Did you try the command? What happened?
Are you on the windows OS? You could write a .bat file to hold the jar file command for repeated use.
Add a More command at the end to hold the command prompt window open after the jar command executes so you can see any errors and copy them to post here.
Here is a batch file I use to create a jar file:
Rem Create the ExtractBoards.jar file:
set JarName=%DEV_HOME%\JavaDevelopment\Misc_Projects\G etHandsFromHTML\ExtractBoards.jar
cd %DEV_HOME%\JavaDevelopment\Misc_Projects\GetHandsF romHTML
D:\Java\jdk126.96.36.199\bin\jar -cmf ExtractBoards.mnf %JarName% *.class
Echo ----- Created %JarName% -----
The names in % are environment variables.