I have lots of experience making precision-cuts on sheet-goods with crappy tools, and tools which seem inadequate to others. It has always been impossible for me to hand-guide my trusty old SkilSaw - other than for rough carpentry work to anything below + or - 1/16".
Yes! You can use a circular saw for high-precision work, but I have found the fancy jigs AND REQUIRED saws (Festool and all) much too expensive for me.
For a good straight line, you can [usually] use the factory-edge of a sheet of plywood as an effective straightedge. Caution: You cannot always trust the factory edge to be straight, so you'd better check it to be true, or capable of straightening up the edge by hand!!. If you measure off the blade and set your marks carefully, you can clamp (or screw through both pieces if you can afford to have little holes in the Work) the guide-board to the piece being cut and slide the edge of the saw base along the straightedge.
One caveat is that the saw blade and the saw base must be in excellent alignment; if a problem, you can add an auxiliary fence to the saw-base out of wood and sand or plane it to perfect alignment.
Something better I have found, and certainly faster - if your project involves identical parts to be cut - is carefully making full-sized patterns out of 3/4 ply or MDF and then rough-cut the pieces to be produced oversize. I cut all the pattern edges with a straightedge and router. Then screw or clamp the pattern to the work and use a router with a pattern-cutting bit and trim off all of the waste around the whole piece. Once the Pattern is good, this method is VERY FAST. Good Luck!