Import the video into Final Cut Pro - select "File" then "Import," and click "Files" from the drop-down menu.
Drag the video into the timeline.
Double click on the video clip. This opens the clip in the selector window.
You should see three tabs in the window: Video, Filters, and Motion. Click "Motion"
Click the small triangle next to "Crop" in the window to open the cropping options.
Make sure the cursor in the timeline is on your clip; this way, you can see the changes you're making as you do them in the canvas. Drag the sliders on the options as you'd like to crop. Your options are left, right, top, down, and edge feather. The first four are from which direction you will crop the clip. Edge feather refers to the edges of the clip, which will become more blurred with a higher setting on the slider.