How do I keep Boston ferns during the winter? The ones I had outside in summer are shedding and appear to be dying. Should I cut them back?

Keep your fern in indirect light indoors and away from direct hot sun or heat. Put it where it will get bright indirect light even an unobstructed north window will do. Bostons can fry in hot sun, they really don't need any direct sun to do well. Dapple is fine which is what mine gets. My friends boston is HUGE, it sits on a tall pedestal in front of a large north window with no curtains. It's 5'-6' across and hangs down over 5'. It's beautiful!

You want to keep it watered but not soaking wet. I water once a week most of the time. Although Bostons like humidity, keeping the soil moist is more important. They don't like to dry out completely or even get too close to drying. When the top inch feels almost, or close to dry, it's time to water it well, letting it drain out.

As far as cutting them back, I have never done that, mine stays indoors year round, so it does not lose that many fronds at all during the fall/winter from outdoor to indoor light and humidity change, but you can cut them back to a few inches above the soil.
Late winter or early spring might be the best time to do that.