Just cd to the directory where you have placed build.xml and give the command

ant

to build the document set. This may take several minutes the first time you do it. After that, successive invocations of ant will usually only rerun commands required based upon which source files have changed since the most recent build.

After the first time that ant has been run, you can also request rebuilds of specific documents with the command

ant documentName

where the documentName matches the targetdoc name given for the desired document in the sitemap.

Finally, you can clean up the generated documents, returning to an almost pure input state, by the command

ant clean