Blender Secrets: Bool Tool

(Check the video below the text) Bool Tool is a really awesome cutting add-on that comes with Blender by default. Make sure the box is checked so that it is active in Preferences.

Create an object that you want to use to cut into another object.
Select it, then Shift+select the object you want to cut into.
Press Ctrl+1 to cut.

Now you see that the Cutter object has become a bounding box.
This allows us to see through it, while still giving us the ability to select and move it.

The boolean operation updates in real-time.
The cool things is, that this is a completely non-destructive way of modeling.
You can add a Bevel modifier, set the model to Smooth Shading with Auto Smooth so it looks good.
You will need to repeat that step for the cutter object as well (called the Brush).

If you don't want to learn the shortcuts, in the Option panel under the Edit tab you'll find all the Bool Tool options.

To apply the cut, you can click on Apply Brush or you can apply the B Tool modifier.
After that you can delete the Brush.