Start off by opening whatever you want to make wispy and do whatever you want to it - sharpen it, color it, whatever.
Go to edit on top, then copy it once you're done.
Once you have your picture copied, go back up and click on Effects. From there, go to blur and then motion blur.
The blur window pops up. This is where you choose - put it at 40 pixels (or less if you want less of a wisp) and figure out which way you want the wisp to move by moving the dial around. When you're satisfied, click OK.
Your picture now should look pretty much like this (My blur is going upward.)
Remember the picture you copied earlier? Now it's time to paste it - go to Edit, paste, and then AS NEW LAYER. Make sure it's the layer one, this is important.
Now you have two layers - the background layer is the layer that is blurred and layer 1 is the one you are seeing now - the normal version of your picture.
Go back on top again (where edit, file, view, etc. is) and go to layers. Under that heading, click on properties. A box will pop up.
Again, play around with the things on this window. Test each blend mode. When you find what you like, go to opacity and move it until you can see the wisp as much as you want it to be seen. For mine, I left it on normal and put the opacity down to 44.
Picture looks like this now. You have your wisp. ^^ If you want to put together the layers...
Go to Layers again, then to merge, then to merge all.
Your layers have merged. Keep working on it or save it - you're done! ^^