Ooh! There are a ton of patterns for skirts like this! What you're looking for (on the line art on the back) is one that has "gores", or little triangle panels that go between the regular dress pieces. The cool thing is, if you have to modify it to make the skirt even fuller, it's really easy to say double the size of the gores
Here is one from McCalls (#5274)
Otherwise, there is something like a tulip dress that would work like that. You could use
this pattern (scroll down to #5914) & simply lengthen the one they show. The skirt is really, really easy to make, too - my mother has made like 5 of them for herself~
Otherwise, if you think of a regular "A line" skirt, it's shaped like an "A". Now, where it hits at about your knees, you can make it an even more dramatic shape, kind of like this:
Sorry for my horrible, horrible ASCII artwork... I can sketch out the rough pattern if you needed it
Here is
a good pattern in a similar shape (Butterick #3134) (view A, and you'd just have to lengthen it) That is the one I'll probably use, and just smoosh it to the top
I have to say thank you for the great picture of Sloth, too - I wanted to make the dress for Christmas this year, and couldn't find any good shots XP