If your circular saw leaves the shelf only to trim deck boards or knock down sheets of plywood to rough size, you're underutilizing it. You can also use it as a joinery tool for parts too unwieldy to dado on a tablesaw.
HardWood Fence

Start with the saw

Start by preparing a saw for joinery work. For cleaner cuts, set aside that 24-tooth rough-duty blade in favor of a 40-tooth regular-kerf blade. Then set the blade 90° to the base plate.

Next build the cutting guide shown below. Measure from the left edge of the saw base to the blade (when viewed from the top), add 14 ", and glue the fence that distance from the right edge of the guide. Glue a cleat 90° to the fence on the underside of the guide.

This guide handles most joinery cuts, but can be downsized for smaller jobs. If you're making extra-deep cuts, reduce the fence height from 3/4" to 3/8" to accommodate the saw motor.

To prepare the guide, place it on a piece of scrap clamped to your workbench with the right edge overhanging the bench. With the left edge of the circular saw base pressed firmly against the fence, cut off the surplus 14 ". Now you're ready to start cutting joints.

Waste removal

If you have multiple workpieces to be dadoed the same, clamp them together for uniform cuts and a stable surface to support the guide. Mark where the dado will start, then use the mating workpiece to mark the end of the cut.

Next, adjust the saw blade to cut the dado depth plus 12 " (the thickness of the guide base). Here we're cutting a dado as deep as the mating workpiece thickness. Test cut a scrap piece to confirm the depth.

To begin cutting the dado, slide the guide edge to the left-hand mark, photo below. (Clamp the jig to the workpieces, if necessary.) Place the saw base against the fence and make your first outside cut. Then slide the guide toward the other cut line until the right jig base edge is a blade-width distance from the right-hand mark, and make the second outside cut, photo below.

After trimming the jig to size, the right edge indicates precisely where your blade will cut. Slide the edge up to your left outside cut line.
The saw kerf should remove the left outside cut line. Allow for the blade width when sawing at the right outside cut line.

With the joint defined, now cut kerfs roughly every 18 " to 14 " apart between the outside kerfs, photo below. You can make the kerfs slightly farther apart for softwoods, such as this cedar, or closer for hardwoods, such as white oak.

Slide the jig along as you saw away waste to keep cuts at a uniform depth and provide a stable surface for your saw.

Removing saw kerfs

After the final kerf cut, use a hammer or your fingertips, photo below, to break the waste off as close as possible to the bottom of the kerf. Clean up the leftover waste with a chisel at least 1" wide. Place the chisel bevel-side up on the smoothest portion of the dado and gently shave off the waste, photo below. You'll still have score lines from the alternating-bevel teeth of the blade, but the surface between the marks should be smooth.

A nudge with your fingertips breaks off the waste between the kerfs. Most should split close to the bottoms of the kerfs.
Shave away waste with your chisel, working from the outside toward the middle to avoid possible tear-out.

The mating workpiece should fit snugly into the dado without excessive force, photo below. If it's too tight, shave one side of the dado with your chisel or sand it using a scrap block with 100-grit abrasive on one edge. Then glue and assemble the joint, reinforcing it with fasteners as needed.

Boards overlapping
You may not achieve the perfection that's possible with a router or tablesaw, but a circular saw can cut tight-fitting joints.

Shop Tip

Not all half-laps or notches require a 90° cut, but you can manage any joint angle quickly and easily with a modified version of the 90° jig. Instead of attaching the fence at 90°, angle and center it on the jig base. Depending on the angle, you may need to extend the fence at the front of the jig to help align the saw base. Use a sliding bevel and protractor to set the fence angle. To give the saw a firm footing as you start the cut, move the underside cleat in 1" from the edge so the jig base overhangs the workpiece edge. Then make the cuts between your lines as you would for a 90° joint cut.