I have had too many close calls to realize that fresh slippery seeds do not jive with sharp objects... and the biggest risk to injury was my own patience. After eating the mango take steel wool or a brillo pad (which ever is closest to kitchen sink at the time) and scrub the slippery flesh residue off the husk. This gives a nice clean seed husk, then place on a small plate on countertop and let sit for 3-4 days. The husk dries out (the embryo inside is fine) and gets somewhat crispy, at that point trip the thinnest edges of seed husk with kitchen shears and easily open with fingers. To germinate wrap embryo in paper towel, wet with water and place in semi-sealed ziplock bag for 2-3 weeks, should see a distinct root by then and can transplant to a small pot.
-JoeP450