Very easy. Take a stem about pencil thickness or a bit less and about 3 inches long. Cut cleanly with a sharp knife just under a leaf joint at the earth end of the cutting. Take off any big leaves and put in a mixture of 25% horticultural sand and 25% normal potting compost and put in a shaded coldframe.

It is getting a bit late for doing this but I had good success doing the above about a month ago. They all rooted in about 3 weeks.

You might find my blog on here entitled "taking cuttings" useful

and you could look at this link showing the geranium cuttings I took a few weeks ago

http://www.flickr.com/photos/31559373@N00/5023052652/

There's also a great deal of information on cuttings/plant propagation on my flickr pages here

http://www.flickr.com/photos/31559373@N00/sets/72157600840587482/