Hmm, you’ve been here as long as me, so I feel a little silly for telling you the basics. Excuse me if you know all this already. π³
The way Malcolm suggested works fine for linear fades. If you need a non-linear fade, the easiest way is to use spline points. You can select which type of points to draw from the three buttons on the left side of the curve/automtion editor. Here’s an example with two points.
If you like it better to record level fader movement, and just smooth out the curve afterwards, then try this: First, here’s a curve I’ve recorded.
Open the automation event in the editor, select all points (Ctrl-A or click-drag over them) and choose ‘convert to spline curve’ from the edit menu. As long as the recorded curve is not too wobbly, there shouldn’t be as many points left now. You can move the remaining ones around a little to smoothen the curve and end up with something like this. Or change it drastically from what was originally recorded, of course.