COMMREX.com Help Center:

Step 3: Click "Scan" button in the pop-up window.

Depending on the make and model of your scanner, your pop-up window may appear different than the one to the right, but all TWAIN-capable scanners use a similar procedure (refer to the user manual of your scanner for instructions on setting up and using the pop-up windows for your model). When you're ready to go, click the "Scan" button, and your scanner will scan a preview of your image.

   

Step 4: Click the "Accept" button to complete the scan.

The preview image will appear, and you may zoom in or out, crop the image or otherwise fine-tune the area you want scanned. For a straight image such as the one to the left, it's easiest to just click the "Accept" button, which tells the scanner to make a complete scan of the image.