What quarantine is protecting
The fish in the shop bag looks healthy, but it has just endured capture, shipping and a crowded sales tank, and the problems that travel with new fish usually show themselves in the first few weeks. Quarantine simply moves that risky window into a small tank of its own, where a problem costs you one fish's comfort instead of a whole established community's health. It also lets a stressed new arrival learn to feed without competition before it ever meets your boldest fish. Simple is a virtue here. The fancier the setup, the more likely it gets dismantled between uses, and a quarantine tank that exists beats a perfect one that does not.
The basic build
Choose a modest tank, big enough for the largest fish you realistically buy to turn and rest in comfortably for a few weeks. Keep the bottom bare so you can see waste, uneaten food and anything the fish passes, and so cleaning takes seconds. Filtration is a sponge filter run by a small air pump: gentle, cheap, and able to stay biologically active. Keep that sponge seeded by running it in your main tank or keeping the quarantine permanently running, because an uncycled quarantine tank would stress new fish with the very water problems you are trying to shield them from. Add a heater matched to your main tank's setting, a lid to stop jumpers and hold warmth, and one smooth, easy-to-disinfect shelter so the newcomer can hide while it settles.
Its own tools, its own rules
The tank only works as a gate if nothing leaks through it. Give it a dedicated net, siphon and jug that are never used on your other tanks, and wash your hands after working in it before touching the main aquarium. Feed the quarantine fish last in your rounds for the same reason. Hold new arrivals for a few weeks, watching that they feed keenly, swim normally and stay clear of spots, fraying, scratching or unusual waste. Any symptom restarts the clock and keeps the fish where it is, in the one place where treatment is straightforward.
Moving day, and keeping the tank alive
When the fish has stayed healthy and hungry for the full period, match temperatures, then move it to the display tank during a calm moment and watch the first interactions. Afterwards, clean the quarantine gear, and either keep the tank ticking over with its sponge filter running or store the sponge in the main tank's filter so the next arrival inherits a living filter, not a sterile box. Plants and invertebrates deserve the same pause, since they can carry passengers too, and they tolerate the bare tank perfectly well for the duration. A faint fin mark or changed breathing during those weeks deserves a calm second look before any treatment is considered. For that check, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.