We have borrowed ideas from this site for a few years now. I needed some stalactites (and stalagmites) for my devil's den. I couldn't get good pictures due to the confines of the room (and I'm not great with a camera).
I used wire screen to make the cone shape. The screen can be bent to whatever shape you want after the cone is made. I then used great stuff to cover the cone. I sat the cone on some news paper and started spraying at the tip of the cone. I sprayed slowly - keeping the flow as even as possible. Then I would return to the tip again and spray another row next to the first. The bigger stalactites took an entire can. I started painting using dark brown as a base color. I sprayed lighter colors on until it looked right.