When you pull up the tool, choose the "Create" tab.  When it's on Smooth, use a strength of 0.1 or higher, and when it's on anything else, you probably want it to be about a tenth as strong as your smoothing.  Having it work from the surface (default setting) rather than the screen is generally the best solution.   Don't worry about mistakes, because you can always zero the morph you've made entirely or use the tool on Restore to incrementally erase what you've done.  If you start using it to fix anatomy, try to find a reference or two first, just because it's much easier when you have a clear idea of what you want the surface to do.