Think of it as three easy pieces: THE NECK STRAP
THE ELASTICIZED BACK
and THE FRONT
Cut two of each piece in the photo below: Top 2 pieces - for halter strap - appx. 12 inches long and 2 inches wide (each)
Middle 2 pieces - body of halter top. If your bust measures 36 inches, cut these two pieces about 39 inches wide (to allow for seams and so the top isn't too tight). They should be long enough to cover your belly. **If your belly and/or hips are much wider than your bust, cut a-line shaped pieces rather than squares so that the shirt will not be too snug on your hips.
Bottom 2 pieces - (optional) bottom band. They are the same width as the middle piece (see disclaimer above) and about 6 inches tall - they will be folded in half to form the bottom band of the shirt.
Mark the shape of the plate with a pencil. We are cutting out the curve under the arm. Note how I didn't exactly follow the circular shape of the plate all the way. Cut yours similarly. When you open the shirt front up, it will look like this:
Lay the front down. Take your BACK piece which is still a big square and lay it over top of the front, matching up the bottom hems. Here I have folded down the back at exactly where it meets the underarm curve. See all that red fabric facing you? That is going to become a casing for the elastic at the back. I trimmed mine a little so the casing wouldn't be bulky and huge...
... made sure my casing was wider than my elastic, and sewed the casing. The elastic needs to be about 2 inches shorter than the casing in order to gather the fabric nicely.
Take the top of the shirt front and fold it down inwards to make a casing. This casing has to be wide enough to fit the halter strap through. Stitch the casing in place. View from the outside:
Take the two straps and attach them at one end to make one long continuous piece. Don't worry about being too neat about attaching the ends together. This part will be hidden in the casing we just made.
Finish the other two ends.