The basic quarantine procedure itself is very simple. Where you come into issues is that, like everything else, opinions vary.
To keep it as simple and straightforward as possible -- what you are doing is isolating the new fish for a period of time. Your goals are to 1) observe the new fish for potential diseases/parasites, 2) if necessary, treat the new fish, and 3) protect your existing fish [the ones already in your pond] from anything the new fish might be carrying. Oh, and you're also giving the new fish a chance to de-stress and "chill" -- a stressed fish is more susceptible to disease. #4 reason is actually an advantage -- you get to interact with the fish and they get to know you which [I've noticed] tends to make them friendlier and less skittish, plus they seem to adapt better once they go in the pond
How long to quarantine? That depends on who you talk to. I go for a minimum of 4 weeks. Some say 6+ weeks. Basically, you want to give a disease/parasite a chance to manifest itself, if there is one present. I'm sure there is an incubation period for these, but I figure 4 weeks is long enough for something to show up.
How to do it? What ever works best for you. What to keep in mind is that you don't want to add stress for the new fish, so.......
Container size -- needs to be large enough to accomodate the fish without over crowding. I wouldn't go smaller than 50 gallons. A large aquarium (for smaller fish), a kiddie pool, a stock tank, a large plastic tote.......... It needs to be deep enough so that it doesn't overheat in the warmer months [if outdoors]. And esp if shallow, a cover so the fish don't jump out or get preyed on (plastic mesh is good).
Water quality -- needs to remain pristine. Which means either very frequent water changes or an adequate filtration system. And/or both. And by "frequent," depending on size of the container and number of fish, this could be as often as once a day. Always add a dechlor if you have city water.
And of course, that means testing the water often ("often" is whatever you're comfortable with and depending on what method of filtration/water change you use).
At minimum, an air pump to add O2 and keep the water circulating.
If doing this outdoors, I would have a thermometer to measure water temp, esp. during warmer months.
Feeding -- opinions vary. Feeding very lightly will reduce the amt. of fish waste which will help with water quality.
Have dedicated supplies that are used only for the QT to avoid cross contamination.
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Here's what I do (and from what I've read, I'm on the conservative side): Minimum 4 weeks. I have a large, 300 gal. Rubbermaid stock tank (it's only that BIG because I had to temp. house all my fish one winter -- yours wouldn't have to be that big). I have a DIY trickle tower and a couple of airstones. I keep a few goldfish in there all the time, to keep the filter cycled. So far, I've never had a fish in QT that turned out to be sick, so I let this system run all the time.
IF I were ever to have an outbreak of something, I would have to decontaminate the entire tank, TT, pumps, and all, which would knock out the current bio [you get the idea....]. But I've tried the "frequent water change" method, and it drove me crazy trying to keep ahead of the ammonia.
Oh, and once I set up a large tote in our guest bathtub [son was at college, but wasn't too happy about it when he came home on the w/ends, tee hee] and set up a 2nd tote above it for a gravity water exchange system. I had to be creative and had strategically sized & placed holes drilled. Every 24 hours or so, I'd fill the top tote with treated water. That water would drip into the bottom tote, where the fish were. Then the excess water would drain out and go down the tub drain. It worked great! I could go away for the weekend and Hubby didn't have to worry about anything except adding the fresh water.
