Basically, you stand up and pedal harder.
It takes a bit of adjustment but it's not as bad as it sounds. Somehow this kind of riding feels easier on an SS drive train, though I can't put my finger on why.

I find I can climb moderate hills just as easily on my FG as on my geared road bike, although it gets tiring to keep your momentum up on very long hills. However, past a certain threshold of steepness I can't maintain a reasonable cadence anymore. At that point, climbing on a fixed gear just flat-out sucks. But such hills are rare in my area and I can usually route around them.