1. In a C-H bond, the H is treated as if it has an oxidation state of +1. This means that every C-H bond will decrease the oxidation state of carbon by 1.
2. For carbon bonded to a more electronegative non-metal X, such as nitrogen, oxygen, sulfur or the halogens, each C-X bond will increase the oxidation state of the carbon by 1. (Certain non-metals are less electronegative than carbon, such as phosphorus, silicon or boron, but bonds from carbon to these elements are much less common.)
3. For carbon bonded to another carbon, the oxidation state is unaffected.  So a carbon attached to 4 carbons has an oxidation state of zero.

So unlike metals, which are almost always in a positive oxidation state, the oxidation state of carbon can vary widely, from -4 (in CH₄) to +4 (such as in CO₂). Here are some examples.

(Don’t forget that this is called a “formalism” for a reason. The charge on the carbon is not really +4 or –4. But the oxidation state formalism helps us keep track of where the electrons are going, which will come in handy very soon).

With an understanding of how to calculate oxidation states on carbon, we’re ready for the next step: understanding changes in the oxidation state at carbon, through reactions known as oxidations (where the oxidation state is increased), and reductions (where the oxidation state is reduced). More on that next time.