Once you've squashed your Sugru onto a clean, dry surface like glass, you can dip your fingers or other tools in soapy water and gently smear it in different directions to refine the shape. If you have more than one color, you can smear them into each other for a neat effect, getting a more intricate design than you might if you shaped them separately and then tried to press them together.
I made some pumpkins for our window after making a spider web. I pressed the Sugru into the window, then gently smeared the orange Sugru back and forth until the lines on the pumpkins suited me. I then added really rough features with black Sugru, and refined the shape with a clay tool.
Don't forget to dip your tools in the water often; you don't want it to stick to the Sugru and mess up your design.
Take your time when smearing Sugru to make designs. This technique usually takes lots of strokes in the same direction to coax the various colors to smear as far and as neatly as you want them to. If you're using two or more colors of Sugru, mixing some intermediate colors can help you blend better. If you want the colors to stay bold and separated, no need to mix others. If you want them to blend, mix a little of each so you can have some gradients in between.
And yes I know our window is dirty on the outside.
I didn't have any green Sugru or I'd have made leaves and possibly vines for the pumpkins. :) Thanks for looking!