How can I replace ROBLOX's default animations with ones of my own?

Before sending me links to the ROBLOX WIKI page or the DEV forum on this topic

BOTH the methods given by those sites do not work, they work in studio but not in public play

I've tried for days to get this to work using the instruction they have SPECIFICALLY GIVEN and for some logic defying reason they don't work, they either don;t play the animation at all, or only play the default roblox ones despite me reassigning them in the correct way. It seems that it is impossible to replace them, which is monumentally stupid and frustrating, urgent help as I believe a correct answer will help many other people with the same problem