How do I cook with flaxseed and how does it help my diet?

If you are talking about using flaxseed instead of cooking oil you have to be careful to not heat it to a temperature that makes it into a bad oil. Smoke and browning is a sign it is to that point.
Other than that I do believe you would cook with it like any other oil.

If you are talking about flaxseed itself. I would assume you would just add some to your recipe or substitute it.
Are you looking for recipes you can use flaxseed in?