In the January 2011 issue of Sound On Sound, Mike Senior looked at the best ways to hide audio edits in Reaper (/sos/jan11/articles/reaper-tech-0111.htm). This bonus material gives a step-by-step visual walkthrough of the technique introduced in the 'Advanced Edits' section.
In picture one you can see a raw edit within a sustained pitched note; the waveform mismatch results in an ugly click. To remedy this, overlap the Items to compare their waveforms, as shown in picture two.
Slide the waveform of the later Item to match that of the earlier Item, as seen in picture three.
Now crossfade over a couple of cycles, as in picture four.
If the 'X' of the crossfade looks curved in any way then select straight fades instead using the drop-down fade menu: see picture five.
