Сначала вопросительное слово, например, what? where? + глагол was (для местоимений he,she,it)или were (для всех других) + подлежащее + основной глагол с окончанием -ing. Если вопр. слова нет, то вопрос начинаем сразу со вспомогательного глагола was или were!

Например: Was he reading the book?  What were you eating yesterday?

Формула past continuous-was или were и глагол с окончанием ing

И получается

Were they planning the picnic?