it will probably be the same as other programs

here goes my explaining style

to fade in move the mouse to the start of the audio track and go to the top left of it click and pull across,

this should create something to fade in

and to fade out do the same in the rihgt corner
There should be something called an envelope filter, or tool or something with envelope in the name.
Thats your tool for fades and cross fades.
I dont use reaper so I cant tell you which menu its in.
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I forget what the image of the button looks like but one of the buttons on the track you want to automate should have an envelope filter function. This will light up a light blue line on the track which you can double-click on to make a handle. Then move each handle around to automate each function.