First, it's important to use a sharp blade. A dull blade will cause the saw to bind and veer off course. Second, you'll need to use a straightedge guide. Attach the guide to the workpiece with clamps or tape, and align the edge of the guide with the line you want to cut. Then, set the depth of the blade so that it doesn't cut through the other side of the workpiece. Finally, hold the saw firmly and try to keep your cuts as slow and even as possible.