The bootflag cannot currently be removed from the Canon 5D3. You should consider this before using the custom firmware to enable the bootflag.Many thanks to Charlie Loc (correct spelling?) for creating it.
Another installation video.
This one from Dugdale posted in the Beginners Guide (http://www.magiclantern.fm/forum/index.php?topic=5520.msg45911#msg45911).
This video is shows installation on a camera that has never had Magic Lantern installed and even needed an update of the Canon firmware first.
In addition to the above two videos, I found this one helpful when installing ML with raw capabilities on my 5d3:
Videos were very useful to me, thanks to the authors and the posters!
Spending hours fiddling since ML successfully installed!.. ;D
Regarding the last video referring to www.cinema5d.com,
Just an extra info, I am running OS 10.9 (mavericks) on my mac mini, and MacBoot downloadable here http://www.zenoshrdlu.com/macboot/macboot.html
also needs the mountain lion fix (http://www.zenoshrdlu.com/macboot/macbootcommand.zip) in order to have permission to make your SD card bootable. (To open these "dangerous" & "unknown" applications in Mavericks, remember to right click (= ctrl click) to "force" open them.)
Enjoy!!
5D3 - CF SanDisk Extreme Pro 16 GB