Due to the shirring the fabric is really stretchy but gathers at the end of the shirring giving the rest of the fabric a good deal of fullness. Because of that you really only need to have a cut of fabric that is your bust size. After prewashing you would then stretch this fabric around you and pull it tight (especially if you intend it to be strapless) if there is any excess mark where to sew/serge and you cut the excess. Go ahead and sew it up! What I did - to add the straps - was to purchase a length that was about 4" - 5" wider than my bust measurement. After preshrinking ( and due to the mentioned gathers) this gave me a little over 10 inches that I could use to make straps.
since these are just simple easy to wear sundresses, I kept it simple and cut two 5" wide lengths of that I then folded in half and half again (more similar to making purse straps then dress straps) and then stitch slightly less than a 1/4 inch in around around both sides. I like the thickness this brings to these straps. After that you would put the dress on and pin the straps into place where they fit you best. I made sure to have one of the bras I would normally wear on and pinned the straps so they covered them up. Then I sewed them on using my machine (see picture below) making sure I stitched over one of the shirred lines disguising the new added stitches unless you are close up. It is hard to tell in the photo below but if you look really closely you will notice the extra thickness of the thread in the area I sewed on the strap.
And lastly a picture of Sewing Helper protecting the dresses!