It is generally an automated task of the ant build tool or similar, but you can manually create the war file:
Open a command prompt and browse to your (unziped) app dir, lets say it is in ROOT folder of tomcat dir, so go to <Tomcat_home>\webapps\ROOT
Use the java archive command 'jar' to bundle up your application
jar -cvf mango-1.10.0.war mango-1.10.0
In English: "compress everything in this directory into a file named mango-1.10.0.war" (c=create, v=verbose, f=file)
You can find addtional info here:
http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/WCC3.html