What this consists of is usually an old pill bottle with a rolled up sponge in it, with holes punched in the top and bottom of the bottle. You soak the sponge, shake out the excess water, dry it off, and plunk in into your instrument case. The moisture slowly permeates the case and keeps the humidity in the 40-60% range, which is, theoretically, ideal.
Such devices are good, but require frequent attention. A good partner with these is a small hygrometer, which tells you what the humidity is inside the case.
There is a commercial version, made by Oasis (see the images) which has in it water-absorbing beads. You fill it with water and cap it, and the moisture seeps through the osmotic material of which the thing is made.
This device is hooked to the strings and put inside the instrument, the notion being that the body needs the treatment more than the neck or headstock, it being thinner and more prone to cracking if it dries out.
So I got one of these to see how well it worked, and it keeps things going for a week or two without needing a refill, which is much longer than the pill-bottle system.
As you can see from the picture, the device isn't large, about the side of short , fat, cigar.
You open the case, remove the humidifier, play, then put it back.
Most recent excursion to The Lehrer, at the open acoustic jam, I got there, got a beer, unshipped the uke, and in the doing of that, dislodged the humidifier without noticing, whereupon it fell into the bottom of the uke.
I played the whole session without knowing it. At the end, when I was packing up, I realized what had happened.
So much for mindfulness ...
Though I have to say, it didn't seem to hurt the sound too much.