This is a compound tense, so two verbs are required: the main verb and the past participle of whatever verb you're using. The past participle does not have to agree in gender or number with the subject. The formula is:

HABER + PAST PARTICIPLE OF VERB

Some examples:

1. I would have helped you. = Yo te habría ayudado.
2. She would have bought the book. = Ella habría comprado el libro.
3. We would have seen him. = Nosotros lo habríamos visto.

Irregular past participles:

Some past participles are irregular in all the perfect tenses.

1. For -er and -ir verbs, if the stem ends in a vowel, a written accent will be required.
2. Some verbs are irregular in the past participle form, so these verbs are considered irregular in the perfect tense. Here are the most common irregular past participles: