Windows and MacOS: download and run the installer file. Keep pressing <OK> and <Next> until done.
MacOS: depending on your security settings, you may see a warning message...
Linux: run sudo ./Jazz-Plugin-1-5-1.sh. You may also want to setup a Software Synth.
You may need to restart your browser.
NOTE: Browsers without NPAPI support require both Jazz-Plugin and Jazz-MIDI extension.
Get the extension for Chrome v.43+ and Opera v.37+ at Chrome Web Store; for Firefox v.52+ at Mozilla Add-ons.
(while the extension is still awaiting Mozilla's approval, you can install it manually from jazz-midi.zip)
Run npm install -g jazz-midi, or check it at npm.
Post-NPAPI Firefox support. Developers are strongly advised to use JZZ.js from now on.