You form the imperfect subjunctive by first finding the Ellos form of the preterite. Then you drop the -ron, and you add on the endings for imperfect subjunctive depending on the subject.
For yo and ella/él/usted, the ending is ra. For tú it’s ras, for nosotros it’s ramos, and for ellos/ellas/ustedes, it’s ran.
If you’re going to use the nosotros form though, you have to remember to put an accent on the first vowel before the ending.
Ex: viéramos.
I hope this helped!
