According to Thomas Brand, it’s pretty easy:
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Purchase and download Lion from the Mac App Store on any Lion compatible Mac running Snow Leopard.
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Right click on “Mac OS X Lion” installer and choose the option to “Show Package Contents.”
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Inside the Contents folder that appears you will find a SharedSupport folder and inside the SharedSupport folder you will find the “InstallESD.dmg.” This is the Lion boot disc we have all been waiting for.