Take the 3rd plural form (the "they" form) of a verb in the preterite tense. Take off the "ron" from the end of the verb, then add the relevant subjenctive ending, either:
ra (I), ras (you), ra (he, she, it), ramos (we), rais (you pl.), ran (they)
or: se (I), ses (you), se (he, she, it), semos (we), seis (you pl.), sen (they)
It doesn't matter which of the two subjunctive endings you use, they each have exactly the same meaning and tone.