I took old material and cut a rectangle to fit around my waist, and then cut two more rectangles for bust area. I sewed them all together and then added a wool braid to hide stiching
Im sorry the instructions arent 100%... but it gives you an idea
Take a piece of material big enough to cover you, make a slit (white lines) for the cleavage part, and then make hems on both sides. at the top make a hem, but wide enough to fit a braid through. Make a braid and sew it under the cleavage part. now make another braid for the halter straps, and lace them through the shirt.