How can we create motion blur in daylight with a DSLR?
The short answer is: use a long shutter speed. To control this, put your camera into Shutter Priority mode (indicated by a "S" on the dial" and adjust the speed to a relatively long time - perhaps a half a second, a whole second, or perhaps longer.
The longer answer for when it gets tricky: You might find that during the daytime, things are so bright that when you set a long shutter speed, even at the camera's smaller aperture (biggest f/ number) you end up with blown-out highlights or overexposure. In these cases, make sure that you're using the lowest ISO possible. If that's not good enough, you might need to look at a neutral density filter, which is a filter that limits the amount of light coming into the camera. These are often used for longer daytime exposures such as those seen with silky waterfall photos.
Another tip that applies to long exposures at any time is that you'll want to stabilize the camera using a tripod or other device to prevent camera shake.
To extend the shutter speed in daylight use the following ...
Lowest ISO possible to slow down the sensor sensitivity
Smallest possible aperture to reduce light coming in (use aperture priority)
Use Neutral Density (ND) filters that reduce the amount of light entering the lens without changing the colour balance. I use ND8 which slows down the exposure by 3 stops, to provide motion blur in daylight.
You can even use ND10 filters to provide immense amounts of motion blur, right to the extent that any moving objects totally disappear.
Here is an example of my own, using an ND8 at sunset to lengthen the exposure and cause motion blur to the extent that the sea became a mist ... http://www.thetrueshot.com/Photographs/Pages/Sunset.html#26
One of the handy things to keep in a camera bag are 2 high-quality polarizer filters, one circular and one linear. When stacked in the right order (circular between camera and linear filter), you can rotate them to create a variable ND filter. This does depend on the quality of the filters however. Two good filters will extinguish almost all light when crossed at 90-degrees. Poorer quality ones will only darken partially. Contrary to instinct, price and "trusted" brand-name is no indication of polarizer quality. Some bargain-basement generic filters have more polarizing strength than $100-$150 ones.
Using this method you can dial-in the exact amount of motion blur that you want by using your preferred or required shutter-speed, aperture, and ISO. The one caveat is that if your scene has any important highlights in it reflecting off of water or other smooth surfaces, either of the polarizers could diminish their intensity. In cases like this it's usually sufficient to rotate both as one filter to readjust their planes of polarization to allow those highlights to show through again.