To use the Patch tool, select it by clicking on the button in the tool palette. In the tool options at the top of the screen, you’ll see a couple of radio buttons, labelled Source, Destination and Transparent. Click on Source, and leave Transparent unchecked. This way the tool will use the selection as the source for the repair. It’s a lot easier doing it this way, as you’ll see.
Next, draw a marquee line around the mark that you wish to heal. Keep the selection as small as possible, but leave some area around the mark. The tool blends edges to preserve the texture of the original, so it needs a small margin to work in.
Once you’ve drawn a line around the mark, simply click anywhere within the selected area and drag. You’ll see that a duplicate of the selection moves with the cursor. Drag this to an area of clean texture and release. The tool will copy the clean area over the mark, automatically blending it in with the surrounding area. Use CTRL+D to de-select the area when you're finished.