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

ESTAR + PRESENT PARTICIPLE OF VERB

Some examples:

1. They were talking on the phone.. = Ellos estaban hablando en el teléfono.
2. He was cooking the food.. = Él estaba cocinando la cena.

Use of reflexive pronouns

With the progressive tenses, you can place the reflexive pronoun either before ESTAR or attached at the very end of the past participle. If you attach the reflexive pronoun at the end, you will need to add an accent on the second-to-last syllable. Examples:

Irregular present participles:

Some present participles are irregular in all the progressive tenses.

1. For -ir stem-changing verbs, change either the E->I or the O->U
2. For many verbs that have a vowel before the -ER/-IR ending, change the I->Y. Also notte that the verb IR (to go) makes this same change.