Since it's a little late for this year, look this post up in 11 months and use it for those 2010 pumpkins :)
Start by opening your Silhouette and setting it up for VINYL. Note that you can use ANY color vinyl.
Download and open the image that you want to use. I used the bat jack o lantern , pumpkin art, skull carving and pumpkin surprise (although this one took on a life of it's own after my daughter got the carving knife!).
Next, you want to remove the unwanted parts of the vinyl (these are the parts that will wind up being cut out of the pumpkin).
Carefully remove the vinyl from the backing and stick it on the pumpkin. This is a little tricky on a round pumpkin, but you can do it! Make sure it sticks good - especially the edges. After that, use a dry erase marker and trace the edges of the vinyl. (A dry erase marker is really easy to rub off if you make a mistake OR if you have lines leftover.)
Peel off all the vinyl.
Just be careful that you cut out the right parts!