Take a screenshot: Press Command (⌘) + Shift + 3 or Command (⌘) + Shift + 4
Open the screenshot in Preview: Double-click the screenshot file
Crop in Preview: Click the Crop tool (rectangular crop icon)
Select the crop area: Drag the selection handles to adjust the frame
Apply the crop: Press Return (↩) or click outside the selection
Save: Click File > Save (or File > Save As to create a new file)
If using the screenshot editor overlay: After taking the screenshot, click and drag the crop handles in the thumbnail preview, then click Done (or Save)