If you don’t have the time to hang around and haven’t bought a tripod, this technique might be of interest — though landscape purists may be horrified. Simply
INSERT the filter into the top of the holder, only pushing it down a few centimetres (you’ll probably see the bottom of it as a blurred line in your viewfinder). Then meter off the ground (assuming the ground is darker than the sky) before composing your picture. Slowly push the filter down until you are happy and finally, press the shutter.
Very simple, but this can be quite effective when you are in a hurry and the view is a little dull. I use a Lee 0.9 soft graduated neutral density filter (which reduces the exposure by three stops at the darkest point).