Seventies clothes are crazy fun, but let's be honest--polyester can get itchy and trap unsavory B.O. With this high-necked halter, you're combining that '70s shape with a more breathable fabric. Ah, cotton--the fabric of our lives.
Ingredients:
1 T-shirt
Scissors
Ruler
Tailor's chalk
Straight pins
Needle
Thread
Instructions:
Turn the t-shirt inside out, lay it flat, and cut off the sleeves just inside the seams. Cut a 1" strip off the bottom to use later as a drawstring.
Note: If the t-shirt is much too wide for you, now is the time to clip a little off the sides and re-sew the seams farther in.
On the front, cut slight diagonal lines from the base of each armhole to the top of the shoulder (about 3" to the left and right of the neck band), as shown. Then cut a straight line across the top, through both layers.
On the back, mark and cut (through only the back layer) a gentle arc from the bottom of one armhole to the bottom of the other. Remove the excess fabric.
Flip the shirt over and fold the top edge down 1 1/2" against the front of the shirt. Pin in place.
Using a running stitch or backstitch, sew along the pinned area 1" from the folded edge. Remove pins.
Thread the drawstring through the casing completed in step 5, and bunch the fabric around it.
Turn the halter right-side out, try it on, and tie the drawstring ends at the back of your neck. (You can tie a small knot at each end of the string to keep it from slipping out of the casing.)
Variations:
You can make this project a no sew by using the original bottom hem (on a smaller t-shirt) as the drawstring casing at the top of your halter. Cut a straight line from the bottom of one sleeve to the bottom of the other. Flip the shirt upside down and cut an arc out of the back. In the front, leaving at least 8" of hem in the middle, cut diagonal lines to meet the arc. Thread a drawstring through what's left of the hem and tie it on!
Seventies clothes are crazy fun, but let's be honest--polyester can get itchy and trap unsavory B.O. With this high-necked halter, you're combining that '70s shape with a more breathable fabric. Ah, cotton--the fabric of our lives. 
