A clue that can only mean one thing gives the case away the moment the reader meets it. There is nothing to weigh, so there is nothing to get wrong. A planted clue has to support at least two explanations, and when it first appears, the innocent one has to be the better bet.
The innocent reading has to win, at first
Hiding the true meaning is the easy half. The work is in the false one, which has to be genuinely the more reasonable reading at the time the reader meets the clue. A weak mystery builds a straw explanation nobody would believe, so the clue really points one way and the innocent reading is there for show. A fair one builds an innocent reading that a careful, skeptical reader would prefer, on the evidence that is available at the time.
The detective prefers it too, and that is what makes the whole thing fair. She is not keeping a private hunch from the reader. She believes the ordinary account, because at that point the ordinary account is the sound one.
Eliminations come from the page
The middle of the book is elimination. One by one, the innocent readings have to fall, and each one has to fall because of something on the page. A measurement that does not fit. A record that says the wrong date. A second look at the same object that resolves a detail the first look was too far off to catch.
What can never move the case forward is the detective's mood. A hunch is not evidence, and a feeling that something is off is not a clue coming loose. If I cannot point to the line that ruled the innocent reading out, then I have not ruled it out at all, and the solution is a cheat.
A dry pot on a wet porch
A porch has six potted plants, watered every morning. One afternoon a visitor notices that five of the pots are damp and the sixth is bone dry. The reasonable reading, the one nearly anyone would reach for, is that the dry pot drains fast, or sits in more sun, or got skipped this once. Those are all ordinary, and every one of them is likelier, at first, than anything odd.
Then the visitor picks the pot up. The soil is dry all the way down, not only crusted on top, and the saucer under it shows no water ring when the other five do. A pot watered that morning and dried out on top would still be damp underneath. This one was set on the porch after the watering, so it has been there a shorter time than it looks.
Nothing here proves who moved the pot, or why. The look and the lift move one plain fact from a guess to a known thing, and no hunch was needed to do it.
The whole thing in miniature
That porch is the whole discipline in small. Two readings live in the same object, the innocent one the better reading until a measurement rules it out, and every step of it out in the open. Fixed to Death is built on a stack of these, each one meant to look like an ordinary repair before it turns into the thing that settles the case. A patient reader can turn every one of them over early and beat my detective to the end. I hope somebody does.