when i do stiff legs, i end up with 3 or 4 light sets while standing on a riser, lowering the bar to touch my toes, and keeping my legs as straight as possible. i lift through my heels, and squeeze it out at top, then return to get a good stretch.
when i do romanians, i start the set with a deep hip stretch at the bar, take the weight as if a deadlift, and with subsequent reps focus on replicating basically the last 80% of a deadlift, keeping the legs only slightly bent.
i tend to do these on the same day, romanians first and heavier, in place of squats, and coupled with extensions...