Hold the fiddle in playing position.
Turn on a tuner or use a tuning app.
Tune the A string first to 440 Hz or your preferred reference pitch.
Adjust the A string peg for larger pitch changes.
Use the fine tuner on the A string for small pitch changes.
Tune the D string to the A string using a perfect fifth.
Tune the G string to the D string using a perfect fifth.
Tune the E string to the A string using a perfect fifth.
Check each string again after tuning.
Make small adjustments until all strings are in tune.
Stop if a string feels too tight or sounds strained.