okay, i made leopard leggings a while ago (longer ones, but still what you want)
so you take a pair of pants and lay them atop your stretchy fabric (jersey? or maybe lighter). trace around them, up to the calf area where your leggings would end. then cut about an inch (maybe two, depending on how stretchy the fabric is) inside the lines you traced. sewwwwwwww. :]
obviously, you wouldn't use your loosest pants to trace.
(p.s. 16 is NOT plus size! crazy lady.)