Let's start with the heresy: you might not need one.
Everyone already owns a miter gauge. It came with the saw. And for most crosscuts, that gauge is 90% of the battle. If you're breaking down big sheet goods or cutting long boards, put the sled idea away and grab the miter saw or even a circular saw. A sled shines in one specific situation: you make a lot of repeat crosscuts on pieces that aren't cumbersome. Think drawer parts, box parts, small project pieces, batch after batch of the same cut. That's the sled's home turf.
Still here? Good. Let's build one you'll actually reach for.
The only mistake that matters
Before we cut anything, the big one. People build their sled like they're outfitting a spaceship. T-tracks, flip stops, hold-down clamps, measuring tapes on both sides, a cup holder. Then the damn thing weighs forty pounds and lives under the workbench forever. And when it's time to make a cut, what do you grab instead? The miter gauge.
Build it light. Build it simple. A sled you won't pick up is just a shelf.
What you're building
A flat plywood base that rides in your table saw's miter slots, with a fence at the back that's square to the blade. You set your workpiece against the fence, push the whole sled through the blade, and get a clean square cut with a zero-clearance slot supporting the wood fibers on both sides. That's it. Everything else is optional.
Materials
Half-inch or three-quarter plywood for the base. Three-quarter is the common pick; half-inch saves real weight if your saw is a jobsite model. A straight hardwood strip for the runner (or two). Some scrap for the fences. Screws, glue, paste wax. That's the whole list.
How much should you spend? Gauge the expense against how much you'll rely on it. If you travel for your job, a private plane might make sense because you lean on it hard. Otherwise, fly coach. A sled made from scraps cuts just as straight as a fancy one. The wood doesn't know what you paid.
The runner: one is fine
Most guides use two runners. One well-known design uses a single runner and works perfectly. Here's the thing people miss: two runners have to be perfectly parallel to each other and to the blade. That's three alignments that can fight each other. One runner only has to fit its slot.
Think like a stool. Four legs are harder to make stable because there are several paths to misalignment. Three legs are always stable because no two vectors conflict. Same principle. Fewer constraints, fewer ways to be wrong.
Whichever you choose, the fit is everything. It should glide with zero side-to-side waggle.
Fitting the runner: sneak up on it
Cut your runner slightly oversized, then sneak up on the fit. Sandpaper works. A small block plane works. Hell, even a chisel works. I've seen guys in the field use a patch of concrete as a sanding block. The tool doesn't matter. What matters is taking a little off, testing, taking a little more off, testing again. Sneak up on it. You can always remove more wood. You can't put it back.
Assembly
Drop some pennies or washers into the miter slots to hold your runner just proud of the table surface. Put a few dabs of CA glue or double-faced tape on the runner, lower the blade all the way, and set your plywood base down on top, registering it against the rip fence or the table edge to get it roughly positioned. Let the glue set, flip the whole thing over, and drive screws up from below into the runner. Countersink them. Make sure no screw tip pokes through the top and no screw sits anywhere the blade will travel.
Screw a back fence along the rear edge. This one doesn't need to be square to anything. It just holds the two halves of the sled together after you cut through the base.
Now raise the blade and make your first cut through the base, stopping a few inches short of the back fence. That kerf line is now your reference for everything.
Set your front fence on the near edge, square to that kerf line. Drive one screw in the middle as a pivot so you can nudge it, check it with a square, nudge it again. When it's right, lock it down with more screws. Fresh holes every time you adjust. Never reuse an old screw hole; the screw will pull the fence right back to where it was.
Glue a block of scrap behind the fence where the blade exits. That's your blade guard and your reminder about where fingers don't go.
Wax the runners and the bottom of the sled with paste wax and buff it off. It'll glide like it's on ice.
Calibration: how fussy should you get?
Three of the big guides treat the 5-cut method like religion. Cut four sides of a test board, rotating the fresh cut against the fence each time, then measure both ends of the fifth strip with calipers and do the math to nudge your fence into perfection down to thousandths.
Here's my take: people get anal about the angle to the ten thousandth. If I spit on the end of the wood, it moves from soaking up moisture more than that. You're a woodworker, not a damn machine shop.
Square your fence with a good square and go make something. If you're building fine furniture and want the ritual, the 5-cut method is well documented and it works. For everything else, a square is honest enough.
The things nobody tells you
Dust will pack between your fence and the workpiece and throw off your cuts. Leave a gap for it. Use your saw blade as a spacer when you position things, that kerf-width gap gives sawdust somewhere to go instead of wedging under your work.
Nobody talks about storing the thing either. A full-size sled is awkward. Here's a thought: if you build a smaller sled onto the miter gauge itself, you can recalibrate it with a literal twist of the gauge's knob, and the miter gauge's storage spot becomes the sled's new home. Problem solved twice.
And when the kerf slot in the base gets chewed up over time, don't mourn it. That's a wear item. Cut a fresh base or fit a replaceable insert strip and keep going.
Go build it
Light. Simple. Square enough. A sled isn't a precision instrument, it's a shop helper. Build the coach version, keep it where you'll grab it, and it'll earn its place.
Watch the sled series on YouTube: Mike Is Making Stuff - including "you don't need a crosscut sled" and the 25k sled build.