Most Helpful Girl
I'm Korean. And I think what you're talking about is monolids.
Here are some great eye makeup tutorials even if you have monolids on YouTube-
And I recommend Mac Fluidline in Black Track for your waterline. It isn't waterline approved, but I've never heard of anyone with sensitive eyes even having any issues. Also, applying black matte eyeshadow powder over your eyeliner with a flat, thin brush also helps. Or even applying a thin layer of your skin tone-colored shadow below your eyeliner where it meets your lower lid skin.