Quarter round is that small, curved piece of trim that sits at the bottom and front of the base molding and other trim. Its main function is to cover the space where your base trim meets your floor, dressing it out. It's essentially trim for the trim. Cutting quarter round is among the easier types of trim cuts to make because it sits squarely in the corner where trim meets the floor.
Measure the length of molding you need, with the span ending at an outside or inside corner. Transfer the measurement to the back of the molding.
Hold your tri-square along the mark on the back of the molding and draw a straight line across the width of the back. Then make a small mark at the top of that line, over the top corner, so it's just visible on the top the trim.
Set your trim on the saw in the same position it will go on the floor, with the back of it against the fence of the saw, the bottom of it sitting on the platform, and the curved front facing you. Position the small mark at the top of the front so that it's directly under the blade.
Swivel the blade's miter setting to 45 degrees in whichever direction is necessary to get the correct angle. For an outer corner, turn the blade away from the measured span of the wood. For an inside corner, turn the blade toward the measured span of the wood.
Make the cut by squeezing the trigger and bringing it down on the trim in one smooth motion.