Direct your browser to the "Cut MP3" website. (See the Resources section.)
Click "Open MP3" and browse for the MP3 file you wish to crop. Select the file and click "Open" to upload it to the page.
Wait for the website to calculate the file--once it has, a waveform version of the file will appear.
Drag the arrows underneath either end of the waveform to the area you wish to crop. Click "Play" to preview your selection.
Continue to adjust the arrows until you are happy with your selection.
Click "Cut" to crop the file. Select an output destination and select "Save."