Open the screenshot in Preview
Click the Markup toolbar button if needed
Select the Rectangular Selection tool
Drag to select the area you want to keep
Click Tools
Click Crop
Save the cropped screenshot
Open the screenshot in Photos
Click Edit
Click Crop
Adjust the crop box
Click Done
Open the screenshot in Quick Look by pressing Spacebar
Click the Markup icon
Select the crop tool if available
Adjust the selection
Save the file
Use the Screenshot toolbar by pressing Shift + Command + 5
Take a screenshot
Click the thumbnail that appears
Crop it in Preview or another editor after opening the file