For the bodice, measure around the chest, then add four inches. This is your desired width of fabric. Length is up to you (but if it's a dress, remember to widen it toward the bottom so there is ample room for the legs to move). Also allow an extra three inches lengthwise for hems/seams.
1. Fold your bodice in half, right sides together. The fold makes for one side seam. Sew up the other side seam. You now have a tube that is open at top and bottom.
2. Zigzag stitch or serge the top edge of the tube, then fold it down at least as wide as your elastic. (You are making a casing for the elastic). Stitch, then thread your elastic through.
3. The back of the halter has two little loops to hold the straps in place. These are most neatly made by taking two four-inch long pieces of fabric and folding them in half lengthwise. Press. Now fold each side in to the middle. Stitch closed from the outside.
See? Here's the before and the after. Also zigzag the raw ends of the loops (they will be on the inside of the bodice but you don't want them to fray in the wash).
4. The straps are made of two long strips of fabric. You need to cut them long enough to reach from the front of the bodice over the shoulder and to tie at the back. Below is one unfinished strap and one finished strap.
Fold each strap in half lengthwise. To taper one end, sew from the corner on an angle as shown below. You are going to sew the entire length of the strip then turn inside out.
It is frustrating to turn a long piece of fabric like this - it gets all bunched up. Usually you leave the bottom end open for turning. An easier way to do this is to sew halfway down the seam, then leave a gap of about 4 inches. Now sew the rest of the seam all the way to the bottom. Use that gap in the middle to turn inside out. You are only pulling half as much fabric through that way. Once the strap is right side out, topstitch the whole thing, thereby closing the gap.
5. Fold the two small loops in half and pin them midway across the back of the bodice. With the raw edges on the inside, sew into place. Don't worry if you sew through your elastic casing. It helps keep it from twisting in the wash later.
6. Do the same thing with the long straps. Pin them where you want them to go on the inside front of the bodice and stitch into place. Hem the bottom. Done!
And don't forget to add your finished products to my Flickr group (link on the sidebar - just click the multi-colored dress).