Look at a picture of Captain Underpants--preferably in the position you'd like to draw. Find a picture of Captain Underpants online or in the book. A google search produces many results that you should be able to use.
Draw the outline of the head and body (which are basically connected without a neck). Captain Underpants is a pear shape with a narrow top and a round bottom.
Add the legs. They should be represented by two short somewhat rounded stumps. Spend some time getting the toes right.
Add the outline of the cape, which covers the shoulders. Do not worry about texture or details on the cape yet. The cape will be drawn with flowing, wavy lines, similar to a bat wing.
Draw the outline of the arms and fingers, protruding from beneath the cape. The arms are skinny and straight. Spend some time getting the fingers right.
Now start on the details. The underwear will be drawn over the bottom half of the body. You'll draw the underwear with bold curving lines to represent the seams and a dashed line through the elastic band.
Add the texture to the cape. This is represented by equally spaced dots through out the entire cape.
Draw the features of the face. The eyes will be simple dots. The nose looks like a curly C drawn on its side. There will be one eye brow (a curvy line above one eye). Finally, the mouth can be either fully open, or open just enough to show a row of teeth.