Boot into Mac OS X Snow Leopard 10.6 on a 2015 MacBook Air
Unfortunately you can't.

OS X Yosemite 10.10.2 is the earliest OS supported on your 2015 MacBook Air. Mac OS X 10.6 Snow Leopard is too archaic (released in 2009) and you won't be able to boot into it on your 2015 MacBook Air.