mod_v8 is now built and installed by default.
If for some reason it's not, do the following:
From the Dialplan, you simply call it as an application similar to:
At the FreeSWITCH™ console, or some other application/interface execute the jsrun call with the script name as its argument.
jsrun script.js arg1 arg2 ...
These can be accessed as argv, argv,.. in the script
If you're running the script as an application for originate command:
console_log("notice","<Whatever you need to print>");