As with any slight, it takes a very long time to do even to 'satisfactory' never mind well. Personally, with generic slights, one of things I actually find which goes against everything I have ever learned is that practicing too much makes you worse at it: you become frustrated and that never helps and you pick up bad habits and end up generally deviating from doing it efficiently. So the approach I have is to learn it, practice it at every opportunity until I get fed up and it starts to feel a bit more natural even if it's bad. Then I leave it for a week or two and come back to it and I find this helps a lot.
Also, read the instructions again before you practice- often instructions to technical slights are very meticulous describing moves to the extend of overload: 'parralell to the verticle right thumb meeting the middle finger contacting the left index finger's second raised phalanx... e.t.c.'. Clearly detail like this is impossible to take in all at once, although by practicing learning slights this capacity improves, and take them in one by one until each detail works spontaneously.
Perhaps not directly specific to the classic pass, but hopefully helpful nonetheless
Oh, and also- certainly with misdirection there is almost no need for slight of hand

I have been called upon to perform impromptuly (is that a word?) ootw, because I particularly shook someone with it and I had to sort out the deck right under their noses- which is always fun

However this isn't always a good thing- whilst switching decks, blatant deck cutting and the sort are all very exciting to perform if one cannot be bothered with the dexterity- there will always come the day when an eagle eyed cynic has his eyes glued to your hands and no 'Look! A purple flamingo!' will divert their gaze.
Sam
''To go wrong in one's own way is better than to go right in another's.'' Dostoevsky's Razumihin.