How to create super smooth slow motion clips in After Effects that will be edited in Premiere Pro I would like to make one of those 600fps slow motion clips that can are on YouTube. such as this 
                
            . I know it can be done in AE but I cannot figure it out.
As 50ndr33 says, you need to shoot/capture at as high an FPS as possible. The more FPS the better the slow motion effect afterwards.

Kronos is also a great plugin, but as he also states - it has some problems when footage isn't linear. If that's the case, you should have look at Twixtor. It's a bit more advanced (and more expensive) but it's better at handling advanced remaps, and gives you pin-point control.