Take the third person plural of the verb in the preterite tense: Estar - estuvieron
remove the 'on' - estuvier
Add 'a' on the end to form the first person singular in the Imp. Subjunctive - estuviera
if you want to conjugate for other pronouns it is as follows:
Tú estuvieras
El/Ella estuviera
Nos estuvieramos
Vos estuvierais
Ellos/Ellas estuvieran