Use a straightedge guide---there are several on the market. The less expensive varieties provide a straight edge to run the circ. saw shoe(the shoe is the bottom plate of the saw) against. The more expensive models have a plate that a circ. saw is mounted on which runs on the guide.

With either type, standing beside the sheet when ripping the 8' length means you only have to reach 2 feet or less, since all you have to concentrate on is keeping the saw shoe in contact with the straight edge.

The only gotcha with using the guide is you have to allow for the shoe to saw blade distance when positioning the straight edge on the plywood.

Here's what I do. I lay a number of pieces of scrap 2X4's on the garage floor and lay the plywood on that. I set the saw blade depth for a little thicker than the thickness of the plywood. I clamp a straight edge in place, kneel on the plywood, and make the cut. Just make sure you use enough scraps, and lay them the right way (crosswise to the cut), so the two pieces will be fully supported both during the cut AND after you complete the cut.

There is another way to cut a 4 by 8 sheet of plywood with a circular saw, that works OK, but is not necessarily better than the methods outlined above. It is possible to use blocks of wood, and several lengths of 2x4, to essentially emulate a panel saw. The plywood would be leaned against a wall, with 2x4 spacers behind, and several blocks underneath. You need the blocks underneath to raise the plywood off the floor, so the cut can be started. The advantage of this method is that is lessens the tendency of the plywood sheet to pinch the saw blade.

The least expensive straight guide is the factory edge on a sheet of plywood. They are typically very straight (no semantic policing, please). Cut a strip around a foot wide and protect that original edge. If you're lucky, the remainder of the sheet is enough for your project--you won't even have to buy an extra sheet.

Blocking it up on the floor and clamping on a straightedge also works for me. A length of 1x6 MDF is also very straight and stays that way. Make a practice cut part way on a scrap and measure the exact distance from the clamped straight edge to the far edge of the cut to allow for the blade kerf. Knowing this measurement, subtract it from your desired final measurement and make suprisingly accurate cuts in wide panels.

DOES ANYONE KNOW HOW TO BUILD DOUBLE DOORS FOR A MINI BARN?
I HAVE A 10' X 12' MINI BARN WITH A 4' WIDE BY 7' HIGH OPENING
MY IDEA WAS TO CUT 1' OFF A 4X8 SHEET OF PLYWOOD AND THEN RIP IT IN HALF TO TRY TO GET 2 DOORS OUT OF 1 SHEET
IS THAT POSSIBLE
I TRIED THE SAWHORSE RIPPING METHOD(WITH NOONE TO HELP ME HOLD1 SIDE) AND GOT A PINCHED UNEVEN CUT
I THINK I'LL TRY THE CLOSE TO THE FLOOR METHOD
ALSOI NEED TO KNOW HOW TO BUILD THE FRAME SO THEY JOIUN TOGETHER EVENLY
I HAVE 1X4 THAT I WAS GONNA USE AROUND THEPERIMETER OF EACH DOOR MAYBE EVEN PUT A CROOSBUCK ON EACH DOOR
I'M NOT MUCH OF A CARPENTER BUT I'M TRYING
I WOULD APPRECIATE ANY INPUT ON THIS PROJECT

Another method is to buy a sheet of foam insulation. Set your saw blade so that it cuts into the foam, but not all the way through. Lay your plywood on top and cut away with a straightedge as described above. Continue to reuse it until it gets too ratty to do its job.

The most common homeowner mistake I see is setting the saw blade to deep.
For carbide tipped blades the entire carbide tip should just clear the wood. For steel blades 1/8 inch is enough.
Long cuts can wander, and if the blade is set to deep it can be very hard to get back on the line (or stay on it).
The other common problem is not accounting for how the blade may get pinched in the cut.
While not a problem with a small piece of scrap, larger pieces (middle of a 10 foot 2x4) can pinch the blade and produce kickback or simply jamb the blade till it halts.
In most cases 4 supports are really needed for large panel cuts, with 2 on each side of the blade so no pinching occurs.

I have a question; I've never seen it done, but I think it would work: For a side-winder circular saw guide for ripping narrow stock like a 2x4, could a straight custom-made piece of wood be c-clamped(very small clamps) to the bottom of the base plate on the side opposite the blade. It would need rounded edges, and must clear the guard. Obviously it would take time to get the right position and size of the guide, and it must be parallel to the cut. Any feedback would be appreciated.

Sounds possible, but largely improbable.

It could work for a single cut/job, but would be difficult to make to be adjustable.

And, since a table saw can be found for less than $50(used)---makes better sense to buy the TS.