A pronominal verb is a verb which has a reflexive pronoun, that is, a pronoun referring back to its subject. These verbs are easily recognized by the pronoun se before the infinitive: se lever, se laver, se promener, etc.

formation
In the passé composé, pronominal verbs are conjugated with être as their auxiliary. Past participles of pronominal verbs are formed like nonpronominal past participles. Note that the reflexive pronoun (me, te, se, nous, vous, se) precedes the auxiliary.

s’amuser ‘to have fun’
je me suis amusé(e), I had fun nous nous sommes amusé(e)s, we had fun
tu t’es amusé(e), you had fun vous vous êtes amusé(e)(s), you had fun
il / on s’est amusé, he / one had fun ils se sont amusés, they had fun
elle s’est amusée, she had fun elles se sont amusées, they had fun

past participle agreement
It is important to note that, in most cases, the past participle of pronominal verbs agrees in gender and number with the gender and number of the reflexive pronoun, that is, an e is added to the past participle to agree with a feminine subject and an s is added for a plural subject.

Je me suis réveillée très tôt ce matin.
Moi, je me suis réveillé très tard!

past participle agreement: exceptions
The past participle does not agree if there is a direct object following the verb which is a part of the body:

 Je me suis lavée. >>>>>>>>>>> I washed.
Et ensuite je me suis lavé les cheveux. And then, I washed my hair.

In the second example, the direct object les cheveux is placed after the verb, so there is no agreement.

Furthermore, in cases where the reflexive pronoun is an indirect object rather than a direct object, as in the verb se parler (parler à), there is no agreement.

Puis, ma mère et moi, nous nous sommes parlé. Then, my mom and I talked to each other.

negation

In the negative, the ne precedes the reflexive pronoun and the pas follows the auxiliary:

je ne me suis pas amusé(e) nous ne nous sommes pas
amusé(e)(s)
tu ne t’es pas amusé(e) vous ne vous êtes pas amusé(e)(s)
il / on ne s’est pas amusé ils ne se sont pas amusés
elle ne s’est pas amusée elles ne se sont pas amusées

credit: Tex’s French Grammar (merci!)