During the weekend I watch these videos and created my first SketchFlow. You should also read Blend->Help->UserGuid. It is well written and provide step by step instruction on all tasks that you may need. For example, I was facing difficulties in resizing and moving controls but after reading few pages in the user guide everything was so easy to do.
Also a good overview is available at SketchFlow: An Overview (Microsoft).
SketchFlow Demo from Silverlight 3/Blend 3 Launch Event Part 1/2
SketchFlow Demo from Silverlight 3/Blend 3 Launch Event Part 2/2
