Here's what you'll need:
- A water-tight plastic container of some sort. The ones I prefer are 2" across and 1" high made by lacons.com, but any flat-bottomed container with a lid will work. Cost: $.79
- Hydro gel for gardeners. I used Nutrimoist, but any brand will do. Cost: $8 for an 8 ounce bag, which should make about 400 (!) humidifiers. These are the exact same crystals you'll find in the nice (but expensive) Oasis humidifiers.
- A drill and a 3/32" drill bit.
Fill the glass about halfway with water. You need to leave enough room that the crystals can expand. If the crystals soak up all the water add more.
Wait about an hour for the crystals to absorb as much of the water as possible. While you're waiting, grab the drill and drill bit and your container.
If you're using a 3/32" drill bit you'll want about 15 holes. Drill them into the lid of the container so they won't get covered up when the humidifier is in your uke case. You also don't want water leaking out the bottom. Some people like to use film containers, but I don't like them for a few reasons. One is that they're generally not clear, so you can't see when they need refilling. Another is that they're taller than they are wide, so the only way to get good ventilation is to drill holes all over the side, increasing the likelihood they'll leak when they roll around in your uke case.
You now have a humidifier ready to use! When it starts to dry out the crystals will shrink—just open the lid, pour in enough water to fill it, close the lid and towel it off. The crystals should last a long time before they need replacing (they'll last longer if you use distilled water, because they'll absorb minerals from tap water, but since the crystals are so cheap it doesn't matter if you have to replace them every year or so).