As dehumidify a room when the air humidity is too high and condensation forms on the walls? The first thing that comes to mind is the purchase of a dehumidifier and in fact this is a good solution. However, consider that these are generally rather energy-intensive devices and your electricity bill could be affected.
Otherwise one can engineer oneself by making gods DIY dehumidifiers which will have as an active ingredient one of the most common and easy to find elements: table salt, aka sodium chloride, in NaCl chemistry.
We all know that when the salt remains in a humid environment for a long time it tends to solidify (reason why we put a few grains of rice in the table salt). This phenomenon depends on the absorption of moisture from the air, which salt does very well, and can be used to our advantage to dehumidify a room.
Given that the active ingredient is sodium chloride and nothing else, it is a question of choosing how and where to put it in the room to be dehumidified. A simple plastic container, or perhaps more than one, to be placed full of (coarse) salt in the points most sensitive to condensation is perfectly fine for this purpose.
As for the amount of salt required, keep in mind that with about 200 grams you can adjust the humidity of the air in a 4m x 4m room. Considering that a standard package contains about 1 kg of coarse salt, spending less than 1 euro you will have 5 small ones do it yourself demidifiers. Just enough for the whole house.
To increase the action surface of the salt according to its exposure to the air, instead of a container you can use cloth or organza bags with a not too fine texture, which will also be easier to arrange at home. Exactly as you do with natural deodorants and anti-moths in furniture drawers but, in the case of salt, taking care to put a saucer underneath to prevent them from being in direct contact with wood or other surfaces that can be damaged.
The salt lamps work with this same principle, which in addition to illuminating the room with a soft warm light (does not consume much) dehumidify the air. They are generally done with salt from the Himalayas, which is purer but works like the other, and also in this case they must be placed on a support. If you like them you can buy them on Amazon for a few euros: Himalayan rock salt lamp
If the environment to be dehumidified is the cellar or garage, you can also use something stronger than coarse salt. For example, the bi-hydrated calcium chloride, which can be placed in containers and trays in the points where you want to intervene.
That said, when the humidity reaches the point of forming condensation that generates mold on the internal walls of the room (typically near thermal bridges) salt may no longer be enough. In this case the problem must be faced in a more structured way.
And finally, how is humidity measured? You need a hygrometer and if you don't have one you can buy one online. In my opinion, this weather station with barometer, hygrometer and thermometer is perfect to keep the temperature and air pressure under control
If you prefer a reliable digital instrument, I recommend this electronic thermohygrometer
Perhaps you may also be interested in these articles: Anti-mold, from do-it-yourself to ad hoc products; Anti-mold paint for interiors