The miter that came with your table saw may be fine for quick cutoffs, but for accurate and repeatable miter cuts you must use a miter sled. There are fancy and pricey versions available, but you can make a very nice miter sled fairly easily.
Step 1: Sizing the Parts for Your Sled
Start by cutting a piece of 3/4" MDF to a size that will fit your saw. It should be wide enough to at least extend from the blade to a little past the edge of your saw. Later you will be routing a slot for the hold down bolt to run in, so estimate where this slot will be, and make sure the size you select will accommodate a slot from 90 degrees to at least 45 degrees. Be sure you have enough board around the slot to maintain the integrity of your sled.
You will also need a nice straight piece of hardwood for your fence, at least 3/4" thick. The length will depend on the size of the board you cut for the base of your sled. It may extend past the edge of the sled. Use a square to check that the bottom of the board is square to its face.
Two bolts will be needed to fasten the hardwood fence to the base. One will be used as the pivot for the fence and must be long enough to extend through the base and up through the height of the fence. You will counter bore the underside of the base for the head of the bolt, so figure that into your measurement. The other bolt will run in a slot in the base. It must be a carriage bolt because the square part under the head will fit in the slot to allow tightening without the bolt turning. You could use a wingnut with this bolt, but there are a lot of nice knobs available that could make your miter sled easier to use and look a lot nicer.
Step 2: Drilling the Pivot Hole and Routing the Slot
The first step in preparing the base is to drill the hole for the pivot bolt. On the bottom of the base, measure from the edge that will run along the blade 1" in and 1" from the edge where the fence will be. Mark this spot for the pivot. Measure the pivot bolt head height and the diameter of a socket that fits on the bolt head. Use a forstner bit slightly larger than the socket and install it in your drill press. You need to counter bore at the pivot point deep enough to cover the bolt head. If the height of the bolt head is more than a quarter inch then you may want to grind some off to assure a stable pivot for the fence. Set the depth stop on your drill press for the height of your bolt head. Clamp the base to the drill press table and drill the counter bore. Then switch bits to one the diameter of the bolt and release the depth stop. Drill the bolt hole through the base.
We will use this pivot hole as a pivot for routing the slot for the hold down bolt. You will need to use a radius device on your router. I created the one I use from two rods that fit in holes on the base of the router and a piece of plastic that has two pieces to secure the rods and a variety of hole sizes. You could use something as simple as a piece of wood secured to the router with a hole for the pivot.
The radius from the pivot to the slot is not critical. Pick a point where you can make a slot from 90 degrees to 45 degrees. Do the routing on the bottom side of the base. Use the pivot bolt for the pivot of your radius device. Use a router bit that matches the diameter of the hold down bolt. Route a slot from a little more than 90 degrees to a little more that 45 degrees from the pivot point. Use multiple passes to route the slot. Trying to do it in one pass may overheat, dull the bit and burn the wood. Test the hold down bolt to assure it will slide easily in the slot. If it is a little tight then change the radius slightly and reroute to widen the slot slightly.
Next you need to route a place for the carriage bolt head to slide. Measure the carriage bolt head and adjust the router to route both sides of the slot to accommodate the carriage bolt. The square under the head of the carriage bolt may be wider than the bolt and you may need to grind or file them till they fit in the slot. You only need to grind two of them. The bolt needs to be able to slide easily in the slot. Make adjustments until it does.
A side note: notice the device with the crank on the side of the router in the last photo. It is called a "Router Raizer" and it is one of the best things I got for my router. It allows easy and precise adjustments from the top side or the bottom side of the router using the removable crank or the knob. It is designed to work on a variety of router models. https://www.amazon.com/Router-Raizer-RZ100-Precise...
Step 3: Adding the Miter Slider to the Base Board
The next step will be to attach a miter slot slider onto the base. There are a lot of options available, just do a search for "Miter slot slider" or "Miter slot runner" on the Internet. I took the cheap route and just used a piece of hardwood. It should be 3/4" x 3/8" for most saws. I used screws that are designed for MDF, but screws that have threads all the way to the head are good. You must counter sink or counter bore for the screw heads. Drill all the counter bores/sinks and screw holes in the slider. Put a screw in one of the holes and measure the length to assure it won't go through the base.
Measure from the blade to the edge of the miter slot on the table saw and add a little, which will be cut off after the slider is screwed on. Mark the bottom of the base and draw a line. Clamp the slider to the line, then drill and screw the end screws. Place a straightedge against the slider to verify it is perfectly straight. If it isn't, use a clamp to pull it into alignment. Then drill and screw the rest of the screws.
With the blade down, test the sled on the saw to assure it is running smoothly.
Step 4: Drilling the Holes in the Fence
In this step you will drill the holes for the pivot bolt and the hold down bolt. Mark a point 7/8" from the end of the fence and centered on the board. If you want to counter bore the fence for the pivot bolt nut, do that first before drilling the bolt hole. Check your drill press table to insure the drill is square to the table. Use the drill bit that is the diameter of the pivot bolt. Clamp the fence board to the drill press table and check that the board is square to the table. Drill the pivot hole in the fence.
Now temporarily install the pivot bolt into the base and put the fence on the bolt. Turn the sled over and move the fence so you can see it through the slot. Place a sharp pencil mark on the fence on both edges of the slot. Move the fence from one end of the slot to the other while viewing the pencil marks. They should look the same throughout the slot. These marks will allow you to locate the position of the hole for the hold down bolt. Remove the fence from the sled and place a mark centered on the fence and centered between the pencil marks.
Using your drill press again and a drill bit the size of the hold down bolt, center the bit over the mark for the hold down hole, and clamp the fence to the table. Check that the fence is square to the table and drill the hole.
Step 5: Installing the Fence to the Sled
Install the fence on the pivot bolt and install the hold down bolt from the bottom through the slot and into the hole in the fence. Make sure to align the flat sides of the square that were filed to fit, into the slot. Test moving the fence from one end of the slot to the other. You may have to make some adjustments to the bolt and/or the slot. I ended up putting a washer with a large hole that fit over the square on the bolt before putting it through the slot. Once you are satisfied with the movement, use a lock nut or double nut on the pivot bolt and adjust the tightness so the fence moves smoothly and doesn't wiggle. On the top of the hold down bolt put a wingnut or a knob. You may have to put some washers on the bolt or cut it shorter if you are using a knob that the bolt doesn't go through. Make sure you can tighten the fence solidly so it doesn't move.
Step 6: Finishing the Sled
Put the sled on the saw and with the blade down, test that it slides smoothly without anything scraping the saw table. Then raise the blade and run the sled through the saw to cut off the extra base to create a perfectly aligned edge. There is only one thing left to do. Install an adjustable 90 degree stop for the fence. I used a piece of clear plastic, but you can just use a piece of wood. Make it about 1/2" square and 2" long. Using a framing square, align the square to the blade or the edge of the sled, position the fence against the square, and tighten the hold down bolt. Draw a pencil line on the back side of the fence near the edge. Fabricate your stop by drilling two holes from the top and one from the side. The hole from the side should be small enough so that the screw fits tightly. Screw it all the way through the stop. Attach the stop to the base a little behind the pencil line to allow room for adjustment. Screw the adjustment screw until it touches the fence. Loosen the hold down knob and pull the fence against the adjustment screw and retighten the hold down. Now use your sled to cut a board and see if it is square. Adjust the stop until you get a perfectly square cut.
Step 7: Finishing Up
You can use drafting triangles or a protractor to set the fence to a particular angle. I find the best tool to set angles is a digital angle measuring tool. Once I have an angle accurately set, I draw a fine line on the table and write the angle.
It's also nice to fabricate a stop for the fence to cut identical pieces, however, it's easier to just use a block and a clamp.
One last item: This sled is great for cutting an angle on the right end of a board. But sometimes you may need to make a cut on the other end of a board when you can't just turn the board over. For that, you make another sled exactly the opposite for the other side of the blade. This second sled also works as a platform to catch the cut-off pieces to keep them from dropping down to the saw table and sometimes having the blade grab them and throw them at you.
I have been using these miter sleds for years. I may not use them often, but when needed, I'm really glad to have them.
Millett made it!