This guy has a good discussion of the process with picture
http://www.pharmacy.arizona.edu/staff/wunz/NoABSTCS/DoubleFlare.html

I picked up a combination tool at harbor freight that makes all three types for $13.00. http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=40878

The only real difference between a double-flare and single flare tool that you'd find at Autozone (or the hardware store) is the little flare plugs. You can order those for $3.00 each from Eastwood. http://www.eastwoodco.com/jump.jsp?itemID=2426&itemType=PRODUCT
Eastwood also has similar kits - but at around $60.00 each.

The $20-30.00 kits on eBay are the same type tool and I'm not sure that the the extra $7-17.00 nets you any increase in tool quality. They are superficially the same tool, so doubt that they'd perform any better (tube-slip during when using the double-flare tip is the most serious problem).

The next rung up in tool quality is around $75.00 and from there it jumps into the 150-400.00 range for professional quality kits. There's a kit aircraft supply company that has some truly different style flare tools, but they get really pricey. I'm sure the quality is high considering the intended application.

The Harbor Freight tool is a a junky tool, but has allowed me to successfully make around two-dozen double-flares for my new 122S/1800E brake system. The tool requires some patience and practice, but around the time I broke it, I was making some really nice-looking double flares.

My 1971 1800e brake system (installed in a '67 122s) uses ALL 3/16" brake line. The fittings appear standard with 1/2 male connectors and 9/16" female connectors. As near as I can tell, all of the old lines had double-flares - though I would have expected the caliper lines to have bubble flares.