Step 1: Open your image
Take an image where you want to cut something out of its background or remove an element from the whole picture, leaving only the background.
Step 2: Choose the tool
Go to the Select button in the Toolbar in Preview and click on it to reveal the additional tools. Select the tool called Instant Alpha.
Step 3: Select the background
Click somewhere in the background and drag the mouse to choose a larger area. The detection works best where the background colour is quite uniform.
Step 4: Redo if necessary
If Preview has included some incorrect areas, click once to undo the selection. Drag the mouse while holding to increase or decrease the sensitivity of the tool.
Step 5: Hit backspace
With the areas selected, hit backspace on your keyboard. The selected content is deleted. With complex backgrounds it’s better to work in stages.
Step 6: Keep removing background
It’s rare that one or two selections will clear a whole background so you may need to keep selecting areas and deleting them manually.
Step 7: Use the other tools
If you’re left with lots of small, fiddly areas of background, switch to the Rectangle or Lasso tool and draw around larger areas to delete them quickly.
Step 8: Stick with it
When you have lots of tiny areas to remove, such as areas between strands of hair, you will have to spend a while deleting but you will end up with a cut out image.