Select the line tool and press to open the settings.

Select 'Curved' from the type menu.

The 'Points Calcultated Along Curve' selector controls how smooth your curve will be. Lower numbers will use few points along the curve and therefore it will be more jagged...

...using higher numbers will similarly smooth out your curve.

Even higher numbers will smooth the line further, but this might not be apparent in the preview window.

The 'Reparameterize Points' checkbox is also helpful for smoothing out the curve.

You can also select a brush from the 'Stroke' menu to apply to your curve.

When you are finished with these settings, press the 'Done' button.

Press and release the spacebar or to place the first point of the curve guide. This and the following three points will determine the shape of your curve. Don't worry too much about placement at this stage, you will be able to move them later.

Use to move the cursor to another location. You'll notice while this happens a gray line appears, this is simply part of the curve guide. Press and release the spacebar or to place another point.

Move the cursor and place a third point.

Finally place the fourth point. Now you'll see your curve appear.

If you place your cursor within the bounding box of a curve guide point, your standard cursor will change to a move cursor (this looks like a cross with arrows on the ends) and you'll able to move the point by pressing and holding the spacebar or and using to move it.

Release the spacebar or to finish moving the point and redraw the curve.

To complete your curve press , select 'Edit', then 'Deselect'.

Your curve has been applied!