If you haven't done so already, you can go a head and stitch up your side seams. Depending on how you want to finish your elastic, you may want to leave 1" open at the neckline on 1 side seam, or when we get there, you can just overlap your elastic, like I will be doing.
Like for any strap, you are going to cut a long strip of fabric. My sliders are 1/2" sliders, so I cut a piece of fabric that was 2" wide. 1/2" seam allowance on either side + 1/2" x2 for my actual strap. The total length should be about 45".
Right sides together, I serged my strip into a long tube. Using a safety pin, I turned it inside out.
Then I divided my tube into 4 parts - 2 2" pieces, and 2 roughly 18-20" pieces (if your straps will be stretchy like mine, I recommend cutting them on the smaller side)
Using a light iron, press your strip flat (Careful not to singe your spandex!). Stitch your tubes onto the o-rings and sliders. Back Tack, or bar tack these in place. Loop your long strip through the o-ring, and weave it through your slider.
Strap Alternative #1: If you do not want this style of strap, sew two long tubes, any desired width, and about 20" long, baste them to the front only, tie behind the neck, and Voila! - halter straps.
Strap Alternative #2: Sew 4 tubes, 20" long each, any desired width. Baste one on the peak of each cup, and one at each side back seam - tie the straps at the shoulder and turn under the ends. Bow-straps!