I'm lazy! I want instant gratification! I'm going to take the provided contest flats and edit them. You can download them here!
I'm going to use the symmetry tool to make this fast. I only have to draw one side when I use symmetry!
Click on the Y button, or, press this button on the toolbar:
First, I'll make a new layer on top of the contest flats. Click the + button on the layer editor.
On this new layer I'll put down a bunch of white using the paintbrush tool at full opacity. I'm gonna cover up the pants part of the pants, but keep the top!
Then I'll make a second layer, and sketch out my skirt idea. I'll connect my sketch with the lineart on top, using a pencil brush in black. I'm not too worried about making it look nice. I'm just trying to figure out my idea first!