Just make certain the crosscut is a perfect 90 degrees.
Make a cut on a 12" piece then flip the cutoff over and butt it up to the keeper piece. It should form a straight line. If not, adjust the gauge until you get one. A sled would be great right now and it will take a few hours, if you have the time. But that's one of the better miter guages, so either way would work.
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)