How to etch into almost any metal. In this case, a stainless steel YETI style type cup.

DISCLAIMER:
i am not responsible if you kill yourself, someone else, burn your house down, shock yourself, or inadvertently travel through time. You are an adult, so you assume this risk (especially the time travel part) if your not an adult, get one to do this and go play with your trucks and dollies or something

Step 1: Materials Needed

here is what your gonna need:
cotton balls. (q-tips will work, but we are covering a large area)
salt water. saltier the better
tape
decal
etching object (steel tumbler)
power source

OPTIONAL
nail polish
chilled beverage of your choice
good music (Galakticon)
stuffed animal for moral support

Step 2: Prep

clean the metal with alcohol and a lint free cloth. put your decal on. make sure that you are using the negative of your image. whatever isnt covered will get etched. if you dont have a negative, put the positive part on there, paint the object in nail polish, and peel off when it is dry. tape off anything you dont want etched

Step 3: POWAAAA!!!

tape the POSITIVE wire to your object. i used a wall wart that pits out 12V and its kinda agressive. a 9v battery will work. A phone charger might do it, but will prob take longer.

Step 4: Time to Etch

soak the cotton ball in the salt water, attach the negative to it (alligator clips are GREAT for this) plug in your power, and touch the cotton ball to the exposed parts. Roll it in a consistant manner, making sure to cover everything. dont let the bare metal of the clip touch the cup or your gonna have a bad time. take your time, holding it on the object for 3 to 4 seconds at a time. when the cotton ball starts smoking and is turning green, its time to swap it for a clean one. just dunk in the salt water and repeat the process . wipe it off every 3 cotton balls or so. oh, and dont breath the fumes.

Step 5: Time to Strip

wipe it down, and peel your decal off. if you used the finger nail polish, clean it off with nail polish remover. clean it off with hot water and soap, taking care to not scratch it. ENJOY.

here is a link to a video that helped me:
https://youtu.be/-m9oxo0e5l0