A coolidor is a humidor made from your basic plastic cooler - like the one you would take your beer to a tailgate in. This has become very popular for several very good reasons:
First, it is inexpensive. Given how much a 50 quart or 100 quart cooler costs and how much it can hold, it is a very inexpensive alternative to the typical wood/cedar humidors. If you catch end of season sales at places like Wal-Mart or Sears, you can pick up a 120 quart cooler for as little as $50. Now that's a lot of cigars!
Second, it is reliable as a humidor by its very nature. A decent cooler is made to be air tight and is insulated to control the conditions within it. Since they are made of plastic they are also resistant to mildew, mold, and other detrimental things that can ruin a cigar.
Third, it has a much larger capacity than your typical humidor. If you are into buying boxes of cigars for long term storage and aging, then it becomes quite costly to buy a humidor with the same capacity.
A common strategy with the coolidor is to use it for storing and aging cigars long term, but then use a "normal" cedar lined wood humidor for the smokes you plan to enjoy short term.
The cooler itself - When buying a cooler, you should consider several factors. How much do you want to store. As is typical with any humidor, the more full you keep the humidor the easier it is to maintain the humidity in it. Consider buying a humidor that will adequately store the number of cigars you plan to have. You should also look for a cooler that has a good seal. It should be air tight when opening and closing to avoid excessive air exchange. You may also consider buying a cooler that has a drain in the bottom of it. From time to time it is good to allow some limited air exchange in order to release the gases that naturally occur from cigars during the aging process.
A humidifier - You have several options here. One that is popular and doesn't require a lot of maintenance are the silica beads, such as the ones you get at rhshield.com, stored within pantyhose.
A hygrometer - Any one will work. Digital is popular for the size.
Cedar cigar boxes - If you don't have any of these lying around, I’m sure your local merchant does. They will likely give you some if you are a frequent patron, otherwise they will typically sell them for a buck or 2 each. Be sure to get decent quality cedar ones - not the cardboard or paper types.
A small fan - This is an optional component that can be used for larger coolers to keep the humidity circulated well in the cooler.
Before using your new cooler you will need to thoroughly clean it. This is best accomplished using distilled water and a mild detergent mixture. Wipe down all of the inside and the lid with a cloth. After cleaning with a detergent, wipe down again with a clean cloth and distilled water only. This not only helps to clean the surface but also to remove the plastic smell that new coolers have. This plastic smell can impart a foul taste on the cigars stored within it. Let stand open to completely dry the inside. If after dry it still has a plastic smell, place an open box of baking soda in the cooler and close the lid over night.
Once the plastic smell is gone, you'll want to focus on getting the internal environment in a good state for storing cigars. Use a clean cloth and wet with distilled water. Take your empty cedar boxes and wipe down with the damp cloth to moisten the cedar boxes. Avoid wetting any of the paper labels that come affixed to the boxes. You can also use dismantled cedar boxes for lining the bottom and sides with cedar. Wipe down the inside of the cooler. Close the lid.
You can now add your humidifier to the cooler. This is a matter of personal preference but given its size the silica beads are a good option.
After the humidifier has been added, you can add your hygrometer. Again a digital one seems to work best in a humidor of this larger size.
As an option you can add a small fan to the cooler. This will better circulate the humidity throughout the cooler. However, a fan too large that runs to fast will dry out your cigars so be careful about the size you buy. A good one referred to me was to purchase one of the Oust brand room deodorizers made by Johnson & Johnson. This can be bought at most grocery stores. It is small, battery operated, and turns itself off and on so that it is not running all the time. Do not use the scented oil that comes with it!
Once the humidity gets up to your desired RH then you can add your cigars. As stated before, it is best to keep your humidor at least half full to help with the humidity. If you are not purchasing enough for that immediately, then add some empty cigar boxes to eat up some of the volume.