I read about doing this too. 

Here are two websites that detail the water method of starting slips.

http://www.food-skills-for-self-sufficiency.com/sweet-potato-slips.html

http://www.diynetwork.com/how-to/how-to-plant-and-grow-sweet-potatoes/index.html

This is a different method-using soil as the medium for starting the sprouts…would work better for mass production. 

http://camppleasant.blogspot.com/2010/04/starting-sweet-potato-slips.html

I read somewhere else that heat and humidity are key to getting a good crop of sprouts. Seems to me that the soil bed with the window on top to boost the humidity/temperature would be a good idea. Anyhow that's what I'm going to try. 

I know that when they are planted out in the garden, they want very warm soil. The recommended date in Coastal South Carolina is anywhere from April 15 to July 1.

Here's Clemson's bulletin on how to plant, etc. http://www.clemson.edu/extension/hgic/plants/vegetables/crops/hgic1322.html