Ho dear, why would you want to do this boring task by hand?
Probably only for learning or scripting the process.
First you need to compile your .java file into .class file.
Just run your project into eclipse it will compile the class into the “eclipse” directory within your project. You can also compile using javac from the command line but it’s out of the scope of this tutorial.
Then open all your jar library using a unzip software. Remember jar file are just a renamed zip file.
And merge them all into a other directory also merge your own compile code.
Also remember to create a META-INF directory at the root of the project. In it add the MANIFEST.MF text file who should look like:
The architecture of your jar is now finish.
Simply zip the whole directory but do not include the root directory itself only the element within the root directory.
Finally rename the .zip into a .jar file and you’re good to go!