Legal Question in Criminal Law in California

You don't if it is 'old' and thus has passed the filing time deadlines.

If it is still possible to timely file, then you consult with experienced local appellate counsel to see if the procedures or rulings in the original case would provide any legitimate valid legal errors, grounds and issues to justify filing and not be dismissed as 'frivolous'.

Appeals are legally 'sophisticated' pleadings with arguments supported by appellate case precedent and law applied to your case facts.