The most formal method, uses the following formula:
debiera debieras debiera + haber + past participle debiéramos debierais debieran
Debiera haber lavado el coche. I should have washed the car.
debí debiste debió debimos + haber + past participle debisteis debieron
Debí haber hecho la tarea. I should have done the homework.
hubiera hubieras hubiera + past participle hubiéramos hubierais hubieran
Hubiera prestado atención a mi papá. I should have paid attention to my father.
There are many factors that will determine which of these three constructions you should use. There are not only regional differences among the three, but there are differences in tone as well.