The tail board is done. That's the whole trick.
Everything the pin board needs to know is already cut into the wood you're holding. The angles, the spacing, the depth — it's all there. Your only job is to move it from one board to the other. Not approximate it. Not re-measure it. Transfer it.
This is the step most instruction skips over, and the step where most first-time dovetailers lose the joint. They reach for a sliding bevel. They measure the tail spacing and try to mark the pins independently. They're doing extra work that introduces extra error, and none of that error cancels what's already in the tail board. It stacks on top.
The method is called direct transfer. The name says everything.
Before the tail board touches the pin board
Square a baseline across the end grain of the pin board — same distance from the face as the tail board's baseline. Do this before the boards come near each other. It's the one independent mark you make on the pin board.
Then sort out orientation. Which face of the tail board meets which face of the pin board? If you labeled your reference faces during milling, you already know. If you didn't, decide now and write it on both boards. This is not a thing to figure out mid-transfer.
The transfer itself
Hold the tail board so its baseline sits flush on the end grain of the pin board, directly over the line you scribed. The tails hang down over the face.
Three things have to be right at the same time: flush with the reference face, square across the end grain, and sitting exactly at baseline depth. If any one of those drifts while you're scribing, the pins won't match the sockets. Get the tail board locked in position before the knife comes out — clamp it against a bench stop, hold it in the vise, recruit a second pair of hands. However you do it, it cannot move.
Scribe the inner face of each tail onto the end grain with a knife. One clean pass per wall. You're not interpreting the angle or guessing at it. You're copying exactly where the tails already are.
Sawing
Square your knife lines down the face of the pin board with a combination square, but only mark the waste areas — the gaps between pins. The baseline needs a line. The pin walls themselves don't need a face mark; the end grain scribe is the guide at the end grain, and the knife wall is where you're headed below it.
Saw in the waste, kerf on the waste side of the knife line. Same rule as the tails. Cope out the bulk between pins before you chisel to the baseline. The pins will usually be narrower than the tails — that's fine. They don't need to match a specific width. They need to match what the tails already decided.
What goes wrong
Two things, always.
First: the tail board shifts during scribing. Sometimes it tilts slightly. Sometimes it lifts off the reference face. You don't know it happened until the dry fit, when one corner closes and the other gaps at the baseline. One side tight, one side open. The fix is not paring the pins. It's understanding which face drifted, going back, resetting, and re-scribing.
Second: sawing on the wrong side of the knife line. The sockets come out too narrow and the tails won't enter on the first dry fit. Before you pick up a chisel, check your kerf placement. It's almost always this.
The standard
You're looking for a joint that closes with firm hand pressure and stops before it reaches the baseline. Not a loose rattle. Not something you have to force. The same joint that closes in the dry run is the one that closes clean under clamp pressure with glue.
The tail board set the measurement. The pin board received it.