You need a scarf that is wide enough to fit you, length depend whether you want it short or long. I recommend to use stretchy fabric.
Mine is sheer stretchy with lovely paisely prints.

Fold scarf in half, and measure from shoulder to under-bust plus 1.5cm for seam allowance. Mark and cut along.This will be the bodice.

Now, take the bodice, fold it in half and cut along.

You would have two pieces of bodice

Serge or zig-zag raw edges, fold along edges and sew.

Join bodice to skirt, right sides together, leave 3cm for elastic casing ; match sides and pin towards middle,overlap two bodices to form deep V-neck, gather the loose middle. Sounds confusing? hope these pics clarify how-to.

It'd be like this:

Repeat joining as above with the other bodice

Sew both sides up to where you want the sleeve to be

For the back, I overlap and sew along 10cm, but you can leave it as it is as deep-V

Fold the casing downward and pull elastic in

And, here it is..your new plunge tunic.You sure are a beach-chic with this.
Check out my blog for more sewing projects :)