Disclaimers: since it's a large T-shirt only altered by being cut with scissors, it's very loose-fitting and wont take a shape unless you do something to it. That's why I added a belt.
Also, it has a really low-cut back (basically backless)
I had no extra shirts to spare so I just used diagrams.
-Seamless oversized T-shirt (just no side seams)
the step corresponds with the image above it
1. Cut along the red dotted line on both sides.
Here's (below) where the tutorial can differ. I stretched both straps and kept the other to sort of hold the dress up. Flip the shirt so the straps are now going to be in front and behind you. I folded one in and I step through both that strap and the dress, and then use the other strap to make the halter. Like this:
If the dress stays up in the back pretty well without it (try it on and kind of let the back strap dangle) then you can cut it off and continue like this:
3. Cut across the dotted line here.
Viola! There's your backless-halter dress. I'm no pro at tie-dying but if you want to do that, too, you can buy a kit or make your own dyes. There are tons of designs on the web you can try!
And if you're into DIY, Threadbanger.com is the place to be, so look around.