Tara M's tutorial for yoga pants out of a tshirt was really helpful for this.
I followed her tutorial but made the pants shorter (4" below the crotch, adding 1" for the hem).
Instead of pulling the drawstring through, i left it open for elastic. (don't put that in yet tho).
instead of having to make the waistband, i just cut the crotch and bottom lines 3" below the armpit of the original tshirt, making it deeper. this way, you dont show any crack and you can just fold it over to make the waistband instead of cutting one out of scraps. i hope this made sense. you end up with the pair of shorts in this picture.
i took a second large tshirt , cut off the sleeves and sewed it to create one large tube. the tube should be wider than your torso and also longer, so that it will blouse over the shorts. if the tube is the same width as the waist of the shorts, it makes it easier to attach. as you can see, the print on the t shirts didnt show on the inside so i turned them inside out to make my romper plain black.
now fold over the top and create a tube to encase the elastic. after you have done this, attach the tube to the shorts. i didn't take a picture for this, because it's quite simple. the tube is sewn under the waist band of the shorts on the inside. make sure not to sew into the waistband, as elastic has to go in there. if your tube is wider than your shorts like mine ended up, just create two small pleats at the sides when sewing it into the shorts. since it's on the inside and the top blouses over, its unnoticeable. make sure to do it evenly and neat though.
cut pieces of elastic for your bust and hips (or waist, as the case may be) and thread them through with a safety pin. all done! now you have a comfy beach cover up, or just an outfit for hot weather :)