Retiming your video clips in post production can be a bit trick. This is especially true if you have a project that has clips that were shot in many different frame rates. Final Cut Pro X has a great feature called Automatic Speed that will analyze your clip, compare it to what you have your project FPS (frames per second) set to and slow down your footage to match that output. This is especially helpful when you want to make sure your footage looks its best when the speed is decreased.