Two basic ways I know of:
Method A
(1) Split the audio at the point you want the fade to begin (on a fade out) or end (on a fade in). I'm assuming that you are at the end or the beginning of the track here but you can use an extra split point to isolate any section of audio anywhere in the track.
(2) Select the just audio segment you made by splitting.
(3) From the Edit menu, select Audio > Fade.
(4) From the Fade dialog box, select the type of fade you want. Edit its nodes if you like, then hit OK.
The selected audio has the selected fade applied to it.
Method B
(1) Create a volume envelope for the track.
(2) Near the time you want the fade to begin or end, add a bunch of nodes to the envelope.
(3) Select the node points and drag them down appropriately.
(4) Right-click the envelope line anywhere between the nodes and choose a curve type if desired.
