Download the plugin and then put it on your plugins server (YOU NEED 1.16 spigot server but it doesnt matter if you are using plugins like via version for multiple version support but your main server version must be 1.16)
Now the plugin must work fine and when a player joins a server a gui will be openned for him to choose his class (All datas will be saved in
"yourserverlocation\plugins\AbilityClasses\player data"