CSG ~ In just finishing our House, I derived some straight cut tricks.
For short cuts, I use a very old metal Square held against the long edge of a Board. I use the other Square side as a stop against which the Saw Guide butts to the left during the cut.
For longer cuts, I select a piece of Lumber by eye with one 'good' side. I use 2 $0.99 Clamps [or Vice Clamps] to hold that Lumber to, say, a piece of Plywood. The Saw Guide butts up against the Lumber and long, straight cuts are possible. The final trick is to have a decent, sharp Blade on the Saw with the 'right' number of teeth for the material being cut. Then, the Saw won't buck or wander off line.