To form the imperfect continuous you use imperfect tense of the verb <em><u>estar</u></em> + the present participle of the verb that tells what you were doing.
Example: Los niños <u><em>estaban jugando</em></u> fuera y los vecinos <u><em>estaban hablando</em></u> en voz baja.
Translation: The children were playing outside and the neighbours were speaking quietly.