Practice First.
Does what it says it does, but its trickier than you might think.  I wish it was slightly more opaque, or a brighter tint, as it's quite easy to miss pieces of it, especially over dark colors.  If you wait too long to make the cuts, it gets brittle and the edges can lift and paint can get under them.  For very small details I had better results with a fine brush.  Good product, but there is a learning curve.
Rating: 4 stars