1. Tap the thumbnail of the clip you wish to trim.
2. Tap Trim Clip on the toolbar that appears at the bottom of your screen.
3. Your toolbar will be replaced by a timeline bordered by blue tabs. Drag these blue tabs so that only the section of your clip you want to use is between them.
Tips on Trimming
1. There is a red indicator line which will show you where you are in your clip. Once you are on the frame you want to trim to pull your blue tab up to meet that red line.
2. Start dragging the blue tab and the clip will pause with the red indicator line in place, but if you pause the clip before you start dragging the blue tab the red line will jump back to the beginning.