In order to use BetonQuest you (obviously) need to add the "BetonQuest.jar" that you downloaded from Spigot to the "plugins" folder in the server.
Dev Build Documentation Disclaimer
You need the latest BetonQuest 2.0.0 dev build for this tutorial.
You also need Citizens for this tutorial. If you can't afford to buy it you can download it for free. Just give the first paragraph of their Spigot page a good read, and you will find the link to the free download. Also put the "Citizens.jar" file in your server's "plugins" folder.
You also need ProtocolLib for this tutorial. Also put the "ProtocolLib.jar" file in your server's "plugins" folder.
Once you have installed these restart your server!