Its quite simple in afew sentences, if you have an existing tank with fish then they need to be placed into a seperate tank for 2 weeks, see if any illnesses arrise. Usually these tanks are almost bare with the basic needed equipment, heater and a filter. The tank is easiest with a bare bottom to make erecting and putting away simple. I would use a cheap internal or sponge filter that has been kept in the main tank. Once they've spent 2weeks in the QT and providing nothing has arrisen and the same healthy you can move them to your main tank. If theyre sick then treat them in QT and make sure to sterilize the filter after and before returning it to the tank.
Fish die belly upward and rise to the surface. It's their way of falling.