Just download the latest jar from the Spigot resource page, and put it into your server's plugins/ directory.
Security notice
If you're running a server where there's multiple levels of staff (most likely, not the case of your small home server), make sure to give a look to the Security Considerations page before proceeding.
If you plan on using command signs as well, in particular the ones with a price, make sure to also install Vault 1.5+ and a compatible economic plugin (e.g. Essentials).
The plugin is being developed on 1.11, but should work from at least 1.9 onwards.