install nano if it isnt there (text editor), 'pkg install nano'.
or you can create in windows (make sure unix format for file), and then upload it.
To setup the cron there is a cron package inside the package manager, after that is installed, can add to that.
example line for cron for hourly. (although the ui cron package I think makes this easier). Assuming script is called ipcheck.sh and is located in /root path.
0 * * * * root /root/ipcheck.sh
That line would be pasted in /etc/crontab
However the way I suggest is install the cron package, and then in "services -> cron" menu it is accessible.
in the ui set minute to anything from 0-59 (the time past the hour to run) and in the hour, day, day of week, day of month boxes put * in each one. user as root, and path to script in bottom box.
Also 'chmod 700 /root/ipcheck.sh' to make script executable. That should be fine if it is owned as root, chmod 755 will also work although less secure, but given pfsense is usually not a multiuser system it probably isnt a concern.