The technique is the following:
1. You need a zoom lens
2. While you expose your photo, you turn the zoom ring in meantime. If you turn too much the whole picture will be blurred. If you turn slowly you see not much blurring occured
3. Shutter speed. For this you need slow shutter speed, between 1/30 – 1/2s or around that. With faster shutter speeds you see no motion, with slower (1s or more) hard to keep the center sharp. Another step is to use a tripod but in this case the center will be fixed, which is perhaps not always preferable. For higher shutter speed you must turn the zoom ring faster to see the effect.
For this you need less light, preferable to shoot in the shade, small aperture of f/16-f/22 depending on light level, lowest possible iso, and if you have ND (simple grey filter to reduce light levels) filter. If possible try to keep aperture at f/16, if you want sharper pictures, because there is a huge drop in resolution at f/22 (in some cases it can also be preferred).
4. Experiment with different shutter speeds and rotation speeds and measure.
5. Strong ND filter helps to make images with better contrast in lighter environment.
6. If you don’t have an ND filter you can add contrast in post processing.