i've tried doin this before, and i find that the only way it'll werk is if the pant legs are a bit wide too. wut i did was sewed up the INseam of the pants and across the crotch. try using a basting stitch frist, and then try them on until u find exactly how u want them to fit. i sure hope this helps!
they look like a flared shaped, like some jeans have. if you hav a pair of flared pants, and you like the size of the flare at the bottom, i'd suggest using them for a template. to make them stay up on your leg, you could either make a casing at the top, and thread a drawstring through, or measure a piece of elastic to your desired fit on your leg, and sew it into the casing. good luck!
i used the simplicity 5238 pattern (above on a previous post) for my grad a few weeks ago, and made the greyish/black polka dot dress, but in pink with white polka dots instead... and it has a gathered bodice. it was pretty easy to do, and the instructions are easy to follow. it looks great when it's done... so if you wanna make a dress quickly i suggest using this one.
measure the length you want it... and how wide around it needs to be... then depending on how poofy you want it to be, multiply the measurement of how wide you want it to be by 2 or 3.. or more if desired to be more poofy. sew all the tooling pieces together at the sides, then machine baste a row of stitching 1/2" from the top edge, machine baste a 2nd row of stitching 1/4" above the other row. gather till it fits to where you are attatching it. and then sew it on. good luck! hope this helped!