in your kettle installation directory are several batch files, among them spoon.bat, pan.bat and kitchen.bat. Spoon is the UI you already know, pan is a command line tool to run transformations (.ktr files) from the command line and kitchen is a command line tool to run kettle jobs (.kjb files).
for a simple schedule create a batch file that calls either kitchen.bat or pan.bat (depending on whether you want to run a transformation or a job). Then use the windows task scheduler to run your batch file with whichever schedule you want.
this for instance would run a kettle job, use basic logging and append the log content to a logfile
kitchen.bat /file:"c:\etl\my_first_job.kjb" /level:Basic > c:\etl\logs\logging_for_my_first_job.log
this is of course for windows. If you run kettle on linux, you can use cron and the respective .sh files in the kettle installation directory (pan.sh or kitchen.sh).
as kettle stores shared database connections in the user profile, make sure the user running the scheduled task has those connections in his profile, otherwise your transformations would fail.