i keep my back as straight as possible for stiff leg deads. i do the same thing for regular deads as well (you should see how straight my back is for regular deads...amazing). other than that, just like iron and gravity said, feet are pretty narrow, i go maybe shoulder width or narrower, knees are locked, and just stand it up.
if you don't feel comfortable off the floor with them, try them in the power rack. i've been doing them (and beast lifts) on the lowest setting. i don't want to go lower for stiff leg deads because i feel like i'll start to round my back.