Step 11: Draw in Ferb's shirtsleeves. These begin under his collar, wrap around the guides for his arms and end slightly inside the rectangle shape.
Step 12: Using the guides set earlier, draw in Ferb's arms by fattening up the lines themselves. Make sure to draw his fingers round like little sausages.
Step 13: Ferb wears his shorts up high, so draw the top part of them right below his shirt button. Make his torso part a bit skinnier than the original rectangle and draw the leg portion of his shorts so they end about halfway to his feet.
Step 14: Under the legs of Ferb's shorts, draw in his cuffs as rectangles with rounded corners. Give Ferb a belt, too, by drawing a series of rectangles near the top of his shorts.
Step 15: Tighten the shapes of Ferb's legs by again fattening up the original guides. Inside each of his legs draw two curved horizontal lines for his socks.