As others have said, there are a) several ways to do this, but b) the best choices will depend upon whether this is a single (or few) shot exercise or whether you envision other DIY projects where perhaps greater accuracy is needed. You can certainly make your own straight-edge jig or use the kind of straight-edge clamp mentioned above by Joe Fauty. I use something similar from Rockler which is a two-piece straight-edge with a track. There is a mating piece that fits into the track and that fastens to the circular saw (I also have one attached to a router base). Each of the straight-edge pieces are about 4.5 ft long, so for cutting across a 4x8 sheet of plywood, you only need to use one. The two pieces butt together end-to-end to give a 9 ft. straight-edge for ripping the plywood in the long dimension. An obvious advantage of having the straight edge in two pieces is its easier to store two shorter pieces and a 9 ft. piece can be pretty unwieldy in tight spaces.
Rockler (with whom I have no connection other than as a customer) also makes a dust hose system that attaches to the sawdust discharge spout on most circular saws and plugs into a shopvac. It does a pretty good job of keeping the sawdust under control. I've found that using a 2 inch thick sheet of 4x8 rigid foam on the floor as a cutting surface works well - at least for making the initial cuts.
- Rich
