This section explains how to use fades and cross fades
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Arranging.61.html Snippet of that document is below:
To create an automatic crossfade (real-time)
1.
Click the Track view Options menu and choose Auto Crossfade.
2.
Click the Track view Options menu, point to Crossfade Type > Default Crossfade Curves and select the desired default crossfade curve types.
3.
Select and drag an audio clip so that it overlaps another audio clip. You should overlap the clips by the length you want the crossfade.
4.
When you have the clip positioned where you want it, release the mouse button to drop the clip.
The Drag and Drop Options dialog box appears.
5.
In the Drag and Drop Options dialog box, check the Blend Old With New check box and click OK.
6.
The two clips now overlap with a crossfade, looking something like this:
Basically, once you enable the auto crossfade from the track view options menu an automatic cross fade will be placed on overlapping clips. You do this with the take lanes collapsed (not visible). Now if you put your cursor over the edge of a clip that is overlapping with another clip a cross fade will be created between the overlapping parts of the clips. If you want it to coss-fade differently you click and drag the fades directly to whatever you want.