Well you gotta know few things, first of all sinusbot wrapper, which is main wrapper/function around all of the script which includes web config.
You gotta know few ways to use your script, which most common are on event, like event on connection, chat, also the function can be executed each x amount of time.
Then you gotta know couple sinusbot functions like setDescription(); which will change the description of an user.
You have to know abit of javascript usage of loops, like forEach or for loop.
I suggest you to use engine.log() function to see your results in console, also read the documentation and if you dont know how to do something simply take a look into other script that resolves similiar problem.
https://sinusbot.github.io/scripting-docs
The simpliest script which would change description of every client that just connected: