1 Make sure your hair is long. If it is not, consider growing it out and putting in cute seashell barrettes or green ribbons (like seaweed) in the meantime. Either way, make sure to have healthy glossy hair. Never look like you have just stepped out of a salon, because mermaids don't go there. Never apply mousse or gel to your hair either, it will look unnatural and fake. If you want, you can also use kool-aid to dye your hair in oceanic colors, like green and blue. just remember not to add the sugar or use the sugar free kind. sugar free kool-aid has artificial sweeteners in it.
2 Always wear aquatic jewelry, which makes people think of the ocean. You can buy these things at Claire's or even Dollar Tree! If you want really realistic aquatic jewelry, you should look on Etsy.
3 Paint your nails blues and greens, or mood changing nail polish. Or go natural with a clear coat of clear polish.
4 Wear blues, greens, and purples. For bottoms you should wear long flowing skirts or pants, never shorts or leggings. It will give the impression you're hiding something on your legs, like scales. You can wear any kind of top, but make sure it doesn't have any brand names on them.