You need a lot of light to shoot at high-speed; outdoors is best, but I've also had success with 500W work lights (you can pick them up for as little as $10 at major hardware stores).

Technically, you're shooting at high-speed, not slow motion; you have to do the slo-mo effect yourself in editing, by slowing down the framerate. Players that don't support high-speed video will sometimes lag, which might make it appear to be "slow" and/or choppy depending on the speed of your computer, but that's not the intended effect. For example, if you shoot at 240fps, you can slow that down to about 24fps for a 1/10th speed while maintaining smooth playback. Many people do this with standard cameras, but at a 25-30fps typical framerate, even half speed is rather jerky. You will also want to either stretch the audio (or simply remove it), otherwise it will completely lose sync with the video.

I don't have any easy-to-use recommendations for editing; I mostly use ffmpeg or Avidemux, which are some of the few that can edit without losing quality, but neither of these tools is particularly easy to use. Perhaps I'll make a guide if there's enough interest?