I use a scribing tool and Dymo tape (the plastic tape you make labels out of). Walmart carries it. Just put the sticky side down on the surface along where you want to scribe and make several light passes until its to your liking. Here's the scriber I use.
http://www.ebay.com/itm/Bare-Metal-Foil-Panel-Scriber-tool-/221132789042?pt=Model_Kit_US&hash=item337c8a9532
I sometimes use a sharp Exacto knife if the line is small, such as meeting two lines on the leading edge of a wing. This particular application is actually easier with the Exacto. Hope this helps,
Joe