Instruments should be built at a lower relative humidity (RH) than they will be at in normal use. This is generally considered to be 45% which I believe gives around 8% moisture content in the timber. Relative humidity is a term used to describe the amount of water that air can hold at a given temperature. As temperature drops, the RH increases until the dew point is arrived at which is when excess moisture in the air (the air being at 100% moisture content) condenses out. There are plenty of posts here on the subject if you search for them...but start with the Robert O'Brien one on RH. El cheapo RH indicators vary in accuracy, but if you search you will find how to calibrate an RH meter using a wet and dry bulb thermometer. I borrowed one from the local high school science lab then used an RH calculator which I googled. It is worth calibrating hygrometers at the RH which you want to measure the most accurately...in our case 45%. There are a few ways to control humidity. One is with a dehumidifier, but these use quite a bit of electricity. There is a domestic one which uses less power than the one I have, But I can't remember...bla bla, just do some research. The other way is to heat the air, so that the RH drops. But of course if you heat a room which is not well sealed, eventually the hot air will stabilise to the moisture content of the air outside.
One reason that low RH body assembly is essential is that instruments can be designed to absorb moisture and expand a little. But if they shrink..that is a recipe for disaster. As for necks, remember that the fingerboard is a much denser timber than the neck wood, so moisture absorbtion and the consequent wood movement is different for the two species. A simple, not so accurate hygrometer can be made by gluing two quite disimilar veneers together in a rectangular strip, mounting one end of the strip in a block of wood with a board at the back and different RH's marked on the board. The strip bends as the different woods absorb moisture at different rates.
https://www.youtube.com/watch?v=Y_jaWKEGhoc
https://www.youtube.com/watch?v=XhlyzoeA_7E