SDs or super deformed characters are a common image found in manga. The basic frame of the SD is a big head, big arms, big feet, and squished body. Some manga artists, though, like to leave the head big and make the feet and arms small. (Example shown below) Otherwise, shown above is the basic SD shape and an SD example with closeups of various sections.
The fun thing about drawing SD characters is their flexibility. You can make them as detailed or as simple as you like. Above are several examples of parts of the SD body. Notice how it can mimic real anatomy but can be reduced to much simpler lines. The simpler the lines, the more comedic the character becomes.
When drawing details, stick to only illustrating the basic lines and maybe one or two lines of folds. If you make too many lines, it becomes too realistic looking. When in doubt, stick to the KISS method (Keep It Simple Silly).
SD’s also has an anatomical rule. That is the length of the arms should reach only as far as the top of the head. Even though in real life, humans can extend their arm over and around their head, if done on an SD, it doesn’t look right and completely takes away the SD cute factor. All in all, SDs follow the same basic rules of anatomy, just dramatically simplified.
Also, SDs tend to get away with being able to express extremely exaggerated emotions.