For the look on the left use dark liner, a nice mossy green and for the illusion of scales use a piece of fishnet (hose or otherwise,) hold it tightly to your skin and use a shimmer foundation to paint over it, making sure not to move the net. When it dries you should be left with some scales.
For the look on the right use lots of shimmer, try to get it slightly paler than your usual tone (it's Halloween go for it!)
Buy some crystals or sequins (make sure the bottom is flat) and use eyelash glue to stick them on.
There are of course other make up looks to try so practice and see what looks good on you. You can always take a little from each look and come up with something truly splashtacular!
If you are brave enough to try fake eyelashes you might want to try the Blakeley. They're cute and blue, perfect for a mermaid!
Once you've finished your makeup, don't forget your hair! You'll want to make sure your hair is pretty and wavy or maybe slicked back with extra gel for that wet look. Or if you prefer it up, try a fishtail braid! You can make your own hair accessories by gluing a shell or starfish to a bobby pin or hair clip. You'll want to do this ahead of time so that the glue can dry. You also need to make sure it's not gonna fall off so wear it around the house for a bit. If you're really artsy crafty you can make a tiara or headband.
Blue and green streaks in your hair are also a nice touch; you can dye it yourself or buy a clip in or go all out and get yourself a green or blue wig!
Mermaids probably wear what they find in the sea, so try a pearl necklace or something older and gold, like from a shipwreck.
And don't forget your toes and fingernails, try a pretty blue, green or silver!