Yeah, basically you want to round out the edges underfoot (assuming you're plan is to hit rails with them), it seems like a terrible idea, but the round edge slides better across the rail/box eliminating that catching feeling you get with new skis and also giving the edge a smooth ride so less cracks form. Everyone kinda has their own method to detuning, some use a diamond/metal file, some just use a gummy stone, some use a combination of both, some use a belt sander, and some use a rock...it all really depends on what you're looking to get out of your detune and what you have available to you. Personally, I hit my edges underfoot with a metal file (a finer grit one) in kind of a rolling motion (so put the ski on it's side and use the file perpendicular to the ski, like from base to topsheet not along the edge, and go back and forth while changing the angle) to kinda round it out a bit, then I go over everything with a gummy stone to take out the burrs and smooth everything out (this I run the length of the edge with). But that's just me, like I said some people hate using a file.
A couple videos to help:
prepping skis from Thomas O'Connell on Vimeo.