First, try to make a noticeably human shape and features. Then, move stuff around slightly, add features like horns and such. Making it either visably bigger or slimmer than average adds to the weirdness. Pointy teeth, maybe an odd amouth, maybe a large mouth. I like to make a realistic skin tone, then shift it to be another colour. Eyes should be a colour similar to the skin to tie it together, possibly a solid colour. Basically, utilise the uncanny valley.