Pretty simple and straightforward.
Take a lightweight fabric (I tried to do this to a hoodie and it didn't really work), decorate with sharpies, spray with rubbing alcohol.
From there I let it dry and then washed it as I would normally!
After being sprayed
My sisters shirt
SIDE NOTE:So I learned to not go crazy when spraying the shirt. I added dots to the rest or the eBay shirt and sprayed until the 8 ounce bottle was empty. I wish I would have taken pictures because spraying it so much took all of the color to the bottom and the dots remained at the top. It sounds cool, but trust me, it looked like a hot mess.
SIDE SIDE NOTE:I noticed that you will get different effects with the watercolor look depending on how you apply the rubbing alcohol and how you let it dry. For the sleeves of the eBay shirt I let it dry on the drying rack like you see in the pictures and when I did the body I hung it on a hanger on the drying rack.
For my sisters shirt she sprayed the left side where she created the design and then folded the right side to it and twisted it together to spread the color & then she let it dry flat.