first determine the angle ....
You must determine the angle first. Then you bisect it, then divide it into two equal parts. You will need to know the number in degrees ... which is what you set your miter saw to. For example for a 90 degree corner, the most common, the miter saw is set to 45 degrees, or 1/2 of 90 degrees. A 45 degree corner would have 22 1/2 degrees per piece.
Now is where it gets interesting... there is no setting on a miter saw for 22 1/2 degrees. Miter saws are scaled to cut the amount subtracted away from 90 degrees.
For a complete and through discussion of this go here:
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)