using the mirror

as i understand useing the mirror in blender is commonly used to work wit 1 side of the face while at the same time working on the other. my problem is, i dont know how to edit one side of the face so at the same time i edit the other. would someone please be kind enough to give me instructions on using the mirror.

if im not to clear on this please notify me

First, you should use 2.40 pre builds. You can mirror in previous versions, but 2.4 has the new mirror modifier, which makes this a lot easier.

Apply a mirror modifier to your object. The object anchor works as the mirror, so you may have to move it to see the mirrored instance.

If you do happen to be on an older version, then you can follow this set of instructions: