Click on the document you want to open. If the ‘check-out’ feature is enabled, you will receive a pop-up box asking how you want to open the file. Select the ‘Check Out and Edit’ button, and click ‘OK’. (note: Mac/Firefox users will not receive the pop-up box, so you will need to follow the next step)
Another way to check out a document, even if the ‘check-out’ setting is not enabled, is to mouse-over the document to reveal the checkbox next to it. Then, select the checkbox. The ribbon interface appears at the top of the page. Click on ‘Check-Out’.