Rip guides like this one can be found on YouTube and in other Instructables here. They are very handy for precise ripping of large panels and for trimming things like doors. I am including a couple of modifications, though, that I think readers will find very beneficial. One is a cost reduction. A second is no need for making a precise cut during the construction of this guide. Another is a way to clamp the guide so the clamps do not interfere with the motor housing on the saw. The fourth is two working edges in case one should be damaged.

In the photo you see my rip guide prepared to trim 1/8 inch from the bottom of a door.

Step 1: Materials and Tools

Materials

1 MDF trim board 3/4 x 3 1/2 x 96 inches
Tempered Masonite 3/16 x 15+ x 96 inches
Bevel-headed wood or sheet metal screws: # 8 x 3/4 inch

Tools

Rule
Drill with bit and countersink
Screwdriver
Sawhorses

Lowe's Home Improvement Stores in the USA sell painted MDF trim boards in various widths. These trim boards are very straight. Since the trim board will be mounted with screws to a piece of Masonite almost 15 inches wide after trimming, the piece of MDF does not need to be completely resistant to bending under lateral pressure. The Masonite will add sufficient rigidity.

Have the store rip 15 or 16 inches from the long side of a sheet of 4 x 8 feet Masonite. It does not matter if the cut is not completely straight, but the panel saw will likely yield a straight cut. (My saw's base is wider than some with almost exactly 5 inches between the edge of the base under the motor housing to the nearest side of the saw blade. If your saw's base is smaller, the Masonite could be narrower. But, the Masonite needs to be twice as wide as the base dimension on the motor side of the base plus the width of the trim board. Leave a little extra on both sides to be trimmed away.)

I had a car, not a truck, when I went to the store. I had the store cut the remainder of the Masonite so I had two pieces about 33 x 48 inches each. They may not go into a car trunk, but will slide through a rear car door and not rise above the seats to obstruct your rear view on the way home. (The panel saw at my store is poorly aligned and corners on crosscuts are not square. The operator has tried several times to get the management have the saw aligned again, but has been unsuccessful. Be aware.)

Plans for rip guides like this often suggest ripping about 6 or 8 inches from the long side of a sheet of 1/2 inch plywood. This is aligned along the edge of a sheet of Masonite. That means you may be buying both a sheet of plywood and a sheet of Masonite. One advantage to my version is that you buy a sheet of Masonite and a trim board only, which makes for a lower cost. (Also, if you are cutting the piece that will actually guide the saw, you will need a means of guaranteeing that cut is perfectly straight. That may not be possible in a beginner home workshop. The straight trim board will be a big advantage for getting a straight guide without a large table saw the beginner workshop may not have.)

Center the trim board on the long Masonite piece and clamp with "C" clamps. Support this sandwich with enough sawhorses to keep it from bowing.

Step 2: Attach With Screws

I chose to have the saw base ride on the smooth surface of the Masonite. So, the rough surface will be down on the finished guide. Here you see the two pieces inverted for assembly. I measured so the screws would form a line down the center of the guide. A screw is installed about 1 to 2 inches from the end. Then I placed screws every 8 inches. The heads are countersunk so they do not scratch a finished surface in use.

Step 3: Trim the Excess From Both Sides

Once the Masonite has been attached to the trim board with screws, invert and support the assembly on enough sawhorses that it does not sag.

I positioned the edge to be trimmed away off to one side so that the saw will not cut into the sawhorses and metal roller support shown. Make certain the cord is not restricted. With one smooth movement hold the saw base against the trim board and move the saw the length of the saw guide to trim away any excess.* Notice that I clamped the assembly to the sawhorses with "C" clamps so I did not need an assistant while cutting.

I loosened the clamps and turned the assembly end for end. Then I clamped it again as before and trimmed the second side. That gives me the advantage of two edges that mark the cut. If one is ever damaged, I still have another.

*My saw has no "run out" that I can determine. On some saws, especially lower-priced saws, the blade moves outward a tiny bit when the saw is powered up. Cutting in one smooth motion is to eliminate the effects of run out. I will use this guide most often on cuts much less than 8 feet long. I have decided to guard against any run out by starting the saw an inch or so away from the cut line and move the saw base toward the edge of the trim board as I move the saw forward to begin the cut. I will also move the saw away from the trim board before the blade stops spinning.

Step 4: In Use

One of the frequent problems with clamping a straight edge for guiding a circular saw is that the "C" clamps obstruct the movement of the saw by colliding with the motor housing.* But, the Masonite on both sides of the trim board means I can clamp the rip guide by means of the Masonite on the side opposite of the saw cut, which is yet another advantage of my version. The "C" clamps are far out of the way, and the guide is securely clamped in place. (You can see a piece of 1 x 2 in the lower left of the photo. It is functioning as a cushion piece to protect the finished surface of the door so the clamp does not leave a mark. Also, the left rear corner of my saw's base is cut at a diagonal, which explains what you see in the photo.)

*If you watched the YouTube video linked in the Introduction, the author of that video cut the piece that guides the saw to be extra wide to allow his small saw to clear the clamps. Too much width increases the weight of these guides. The version I made weighs 14 pounds, but provides two working edges. You will need to make your own choice.

Step 5: Trim

Align the edge of the Masonite with the cut line. After the guide has been clamped in place, make certain the cord is out of the way. Start the saw and trim the excess from the panel or door.

Step 6: Finished Cut

The photo shows the bottom of the door after it has been trimmed. Notice how neatly and smoothly the cut edge follows the edge of the Masonite.

Years ago I purchased a two piece aluminum saw guide for ripping. It deflects a little in the middle and I always had to buttress it from the side. Even then, it was not completely satisfactory. This guide from Masonite and an MDF trim board is so much better. When I need to cut something less than 8 feet in length, I just allow equal portions to hang over at the ends and it all works very well.

I stand my rip guide against the wall when I am not using it. After a while it will begin to bow a little. That is not a big problem, but I can also turn it over periodically so the new position takes out the old bowing. After a while I will need to turn it again.

This guide is made to be used with my saw and the blade I currently have on it. A different saw or a different blade can introduce variables and should have their own custom constructed guide. Also, using this guide with a different saw could render it useless for my saw. If you lend your guide, lend your saw and blade, too.

I wish I had known about these rip guides years ago and had used one then instead of making do with improvised guides that often left "C" clamps in the way of the saw's motor housing. I am also glad I was able to make mine without buying two sheets of plywood.