First of all you have to download the pants template here
Once you have downloaded it, open it with SketchBook Pro.
Now let's start!
Select "Steady Stroke" and "Paintbrush" and adjust the size to 5,5.
Clicking on the colors editor, select a very light lavender.
We have to imagine the pants divided in 4 sections so let's start from the one at the bottom and enlarge that part. This will let you draw better.
Now starting near the end of the pants, draw something like deep waves: it must resemble dripping paint.