You can of course buy and install a fully-fledged commercial XML editor with XSLT support, and run this application within it. However, this is beyond the reach of many users, so to do this unaided you just need to install two pieces of software: Java and Saxon (URIs are correct at the time of writing). Neither of these has a visual interface: they are run from the command-line in the same way as is possible with LATEX.