Easy way to do fade-ins fade-outs and cross-fades in Reaper. Basic Reaper DAW tutorial
How do I fade in the audio in Reaper?
If you want to fade in or fade out this video shows you how it's a simple matter of placing the cursor near the top of the edge of the audio so the cursor changes to the fade icon. Then when you drag with the mouse you can create a fade-in /-out or cross-fade.