Unless you take responsibility for maintaining the build script yourself, it is best to regenerate new scripts each time before executing them. This way, changes in folder structure, new files, deleted files, and new plug-ins will be accounted for.
When Ant scripts are executed from within the wizard, a wizard view will appear in the active perspective so that you can see build messages.
When a build is complete, you should see individual JARs in the plug-in and fragment projects, as well as the feature JAR that has the following name format: <feature_id>_<feature_version>.jar
If there is no change to the structure of the plug-ins, you can also run the build by selecting the build.xml file and selecting the "Run Ant" action from the popup menu. The resulting JARs will be identical in both cases.