1. If you're running Lock-On style grips, use the appropriate sized Allen key to loosen the collar bolts before sliding them off the bars. If you're running slip-on grips, squirt some water in under the grip before slipping them both off of the bars.
2. Use a tape measure to measure the length of the bar and the distance between each of the cut marks. Calculate how much you're looking to trim off. Slide your saw guide over the bar and clamp it tightly over the desired cut mark.
3. Carefully saw the bar with someone bracing the opposite side to minimize movement. Repeat on the other end of the bar. Check you've achieved the length required by re-measuring. File off any burrs left from sawing and then reinstall the grips.