measure the base of one of your breasts as well as the height. you want a triangle that cover, don't you. cut out 4 (2 is for lining).
measure around your bum and add 20 cm/8", divide this number in two and cut out a rectangle as wide as your newly calculated number and as long as you want it (don't forget to add about 4 cm/2" for seaming).
now it's time to sew it all together! pair up your triangles and sew them together, wrong side against wrong side. sew on bias tape on one of the sides of your triangles. when you sew on bias tape at the outer sides of your triangles, the sides that will be facing your arms, let the bias tape form a tail which you can either fasten on the back of your slip or knot together at the back at your neck for a halterneck version. measure first how long the bias tape needs to be.
sew your rectangles together, right side against right side, to form a tube.
pin your elastic around your tube, about 2 cm down from the edge, evenly. zigzag.
fold the excess fabric over your elastic and pin it down. place your triangles at the front (duh), let one of the facing corners slightly be on top of the other. ruche a bit at the center for a better fit, if you care to. sew around.
embellish or leave it as it is. done!