That being said, here's how I made my Dax costume.
Step 1: Badge. See my tutorial on DIY Star Trek communicator badges.
Step 2: Clothing.
I don't have time, nor am I a serious cosplayer, so my costume is very basic. Dax wears all black with a turquoise panel around her shoulders. I layered a black high-neck t-shirt (inside out, because it has a design on it) with a turquoise v-neck. If I folded the shirt up, it looked pretty close.
I decided to cut the shirt in half. The only problem is I painted flowers on it (remember when I did that?). I had already decided to get rid of this shirt, so cutting it up wasn't a problem.
I cut it, hemmed it, and ironed on a circle of fabric over the flower. I wish it didn't have the "patch," but I don't have time or motivation to go find another turquoise shirt, so it will have to do.
Step 3: Neck spots.
Dax's main alien feature is her pathway of spots that goes from the middle of her forehead down her entire body, I guess. I studied a few photos before going to work on myself.
I was glad I had a high-neck shirt. I basically just gave myself irregular spots and circles with eyeliner from forehead to neck.
I used my little medicine cabinet mirror to get the peripheral parts of my neck and under my ear. I literally studied a photo of her and did my best to copy the markings.
Bonus: Here's our group!