i'm guessing you mean ramping. i've had multiple people ask me how i did the ramping on some of my recent edits, and truth be told i have no idea how to ramp slow motion. you just have to know how to go to slow motion at just the right moment to give the illusion of a smooth transition into the slow motion.
if you actually mean fading in and and out of slow motion clips, then you don't need to do that either. just have to learn pacing.