If your crosscut sled isn't accurate, it's almost useless.
If you need to square up a piece, each consecutive cut is worse than the last. precision is everything.
I dial mine in to less than 0.001", here's how to do it:
A Combination square or machine square
A tape measure
An electronic or dial caliper
A set of feeler gauges
A pen or pencil
Here are the steps to adjust the fence:
(Thanks and all credit to William Ng from the School of Fine Woodworking)
You need 2 pivot points. One on each end of your fence (screws or bolts)
Mark them with your sharpie and measure the distance between them.
Raise your saw blade above the surface of the sled.
Loosen one pivot point and set your fence square to the blade using a combination or machine square.
Using a rectangular piece of stock, 1/4" or 1/2" plywood or MDF would be great but use whatever you have to hand.
A good sized piece that fits on your sled would be perfect.
Make sure that the first edge is flat against the fence.
You're going to make 5 cuts, one on each edge of the stock.
The fifth cut is for measurements (Turn the piece clockwise as you go).
Holding one of the shorter edges of the piece against the fence, cut a slice away from the right edge as normal.
Thats cut 1.
Turn the board 90° clockwise and cut the next edge, Cut 2.
Do that again with the next edge (cut 3), and the next one (cut 4).
Now turn the piece one more time and line up for the 5th cut.
Cut the 5th piece wide enough to measure it's width with an electronic or dial caliper. I'd suggest 1/2" to 1".
Once you've made the 5th cut, allow the blade to stop and before you lift the piece, mark the top/front (A) and the bottom/back (B).
Now for the math...
Write down any measurements you take in inch fractions to 3 decimal places. (for example 1.500", 1.175", 6.283" etc)
Measure the width of the top of your off cut - (A)
Measure the width of the bottom of your offcut - (B)
Measure the length of the offcut,(the 5th cut) - (C)
Measure the distance between the 2 pivot points that lock your fence in position (D)
Here's the equation - (A - B) / 4 / C x D = E (E = how far your sled fence is out, the error ratio)
(4 represents the 4 cuts you made)
Here's an example:
A = .726" B = .735" C = 8.250" D = 30.75"
So (.726) - (.735) / 4 / (8.250) x (30.750) = (0.041")
Take a set of feeler gauges and find one that's as close as possible to the error ratio (E), 0.041".
In this example, 0.041" is a positive number.
If (E) was a negative number, the right of your fence needs to come back toward you or the left forward, away from you.
If (E) was a positive number then the opposite is the case.
Use a clamped block against the pivot point and the feeler gauge you chose, move the fence at the pivot point. Lock it down and you're done.
I learned this from the brilliant, William Ng from the School of Fine Woodworking. He also has a great YouTube video of the process which you'll find right here.