To shoot slow-motion video, open the Camera app then swipe across the display from left to right if holding the phone vertically, or bottom to top if holding horizontally, until the Slo-Mo option is selected. Record video as normal. When you watch it back, the middle segment will automatically transition into high-quality slow-motion.
slo-mo

Editing

You might want to change when the clip changes from regular speed to slo-mo, so it kicks in at the perfect time for maximum effect. To do this, open the Photos app and find the video. Tap to bring it full screen and then press the Edit button along the bottom, which looks like a set of sliders.

This reveals a timeline of vertical white lines. The slowed-down segment is the series of vertical lines that are spaced out, between two taller white lines. Drag on the taller lines to increase or decrease the length of the effect, then play back the video to take a look; the main white scrubber on the thumbnail timeline below will move with your action. Hit Done when you’re happy with it. This edit isn’t permanent and can be altered as many times as desired.