ok, here is more or less how i made it- i'm happy to answer questions if this doesn't make sense.
First, take your shirt and cut it where the red lines are (except less wonky):
You want to cut real close to where the bottom of the arms are, and then cut it straight across the back. Put the shirt on inside out, pin it so it fits your body loosely, and sew those seams (don't try to make it form-fitting just yet- that comes later).
now, take the collar- just the collar part, detach the excess shirt fabric first- and cut it in half (some of this went beyond my MS Paint skills, so i took a few pictures):
flip it around so that the cut end faces the middle of the dress (where the buttons are) and fold about half your half collar down:
let's all pretend that the scrap of fabric below the collar is actually half of the dress, ok? like this:
except here it's backwards and i can't convince craftster photos to flip it around. sigh.
take your folded collar and fold it over the top of the dress bodice like so:
now unfold the collar, fold a hem in the top of the bodice, pin it down and sew it:
fold the cut edge in:
and sew it down.
Now take some of the excess fabric you cut off the sides, sew a long tube, and cut it in half- that's your halter straps. Attach them them to the top of the bodice.
Ok, so now just put in darts so the thing fits (i put in four in the front and two in the back) and hem the bottom, and yr done! Do not be afraid of darts, they are your flattering friends. There's a little tute on whatthecraft: http://www.whatthecraft.com/tuts/darts.htm
The darts i put in are shaped like this:
ok, happy sewing! if any of y'all make one, i'd love to see it.