Take your image, zoom a bit in (around 400% or perhaps 800%), take your pen tool, and left-click somewhere near the edges of the image :
Then, press at a point somewhere further, like here, and hold your left-mouse button :
Now, while holding down the left-mouse button, go down/up with your mouse, to create a nice and smooth "bow":
You'll see 2 lines with squares at the end appearing. The line on the left determines the bow between this point and the previous, the line on the right should point to the direction of your next spot, in this case, below the knee. When you now create a new spot there, you'll get a smooth line from the middle point, and your new one :
Now, press and hold left-mouse button, and place a new spot somewhere on the lower part of the leg, and drag your mouse till you get a nice "bow" :
Now, the 2 lines with squares at the end will reappear. If you don't do anything with them, and just make a new spot, you'll get a messed up result like this :
So drag the line with the square at the end close to the leg, and then make a new spot :
Always work in small parts. Don't try to completely render your image in 1 try. Render body parts, then press 'selection from path', and delete the area around the body parts, and then save as backup. Here's 2 more images to explain :