use a straight edge clamped to your board you are cutting. ( I am assuming you are talking about long cuts, for short crosscuts, use a miter box). You can use a long level, yardstick, length of steel (angle iron is good), or 2x4 (for wood, make sure to use the thinner dimension, as it is less likely to warp in that direction). You have to figure out how far away from your cut line to clamp it down, for my circular saw its an extra 1 1/4", so I draw that second line the same time I mark the line I want to cut.