Open iMovie and create or open your project
Import the movie clip into the timeline
Click the clip in the timeline to select it
Move the playhead to the point where you want to cut
Press Command-B to split the clip
Move the playhead to the next cut point if needed
Press Command-B again to make another split
Select the unwanted section
Press Delete to remove it
Drag the remaining clips to adjust their order or timing if needed
Save or export your project when finished