I love this technique, but it does take practice.
Thank goodness for digital cameras. I can practice all I want, and throw out the duds.
The trick is a SLOW shutter speed. It should be 1/8, 1/4, or 1/2 second. It has to be slow enough to actually capture the motion. At 1/500 or 1/30 second won’t cut it as your inferred motion will be too slight.
The tricky part is getting the shutter speed that slow during the day. There are several ways to assist in this. Use a small aperture (f/22), allowing for the slowest shutter. Use a polarizer or Neutral Density filter to cut the light, and slow down the shutter.
Once you can get the shutter slow enough… let ‘er rip.
This is a hand held technique, not tripod. So you have to move with the subject. Focus on where the subject will be crossing your path (or use Al Servo) parallel to where you await your subject to pass. Also use your high speed exposure to take multiple photos as you swing with your moving subject.
It’s good fun and a cool technique. Good luck.