Find a watertight container large enough to comfortably hold your koi for a week or two. It must have a rim around the top. Plastic storage containers and kiddie pools typically work well. Simply place the container in the pond, fill with pond water, add your fish, and add medication if necessary. The rim around the top of the bin will hold enough air to keep the container afloat, keeping the quarantined fish and water separate from the main pond. The surrounding water will naturally maintain the water temperature within the quarantine container. If necessary, tie a few lines to the container to keep it stationary and simplify maintenance.
While quarantining your fish, be sure to aerate the water within the quarantine container using an air or water pump. Also, if you cannot place the container in a shady area of the pond, cover the container with a shade cloth, an umbrella, or floating artificial plants. You should also cover the container with pond netting to keep fish from jumping out.
Because the quarantine container does not contain biological filtration, you must monitor water quality with a test kit and perform regular water changes. To perform a water change, simply hook a hose to a water pump or powerhead within the container, and direct the water outside of the pond and away from your garden. To refill the container, move the pump into the pond, and direct fresh pond water into the quarantine container. Be sure to follow the directions on the medication and replenish it after performing a water change.