Pin facing to front of shirt (facing is another piece of fabric that will end up on the inside of the garment to stablize and help keep shaping) - This facing came from another white t-shirt that was stained so I didn't mind cutting it all up:

Draw in shaping with water erasable marking pen (I wanted my shirt to have kind of a keyhole v-neck, but you could do a boat-neck, scoop-neck, true v-neck, or whatever you would like), then stitch about 1/4" to 1/2" away from edging and shaping drawing:

After stitched, clip and trip away excess fabric:

Turn facing to the inside, pin again and stitch about 1/4" away from finished edge to hold in place. I added some decorate ruffles around the edges, but it looks good plain too.
