How to make smooth slow motion?

You can use Time Remapping or a speed change on a clip in After Effects and Pixel Motion blending from Layer > Frame Blending > Pixel Motion https://helpx.adobe.com/after-effects/using/time-stretching-time-remapping.html

You can also change the speed of a clip in Premiere by choosing Optical Flow as the Time Interpolation method. https://helpx.adobe.com/premiere-pro/using/duration-speed.html 

Both options do a good job, within reason, but are slower to render than the other available options. I use Optical Flow frequently when slowing clips down in Premiere with great success. Note, you'll probably get a red render bar when you enable this option, so it's best to render your clip then play back to see how well Optical Flow worked.