Look up the properties of the elements and choose one that works for you. For the purposes of this example, choose carbon.
Determine what powers best relate to carbon. Carbon can be as soft as charcoal or as hard as diamond, so our example hero will have control over his own density.
Name your character. Example: Carbon Man, the Carbonizer, etc.
Determine an origin story for your character, based on their element and powers.
Create a costume for the character, using the pencil and paper to sketch it out. Carbon is generally gray or black, so use those colors in the color scheme of Carbon Man's costume.