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As much sun as possible is key! A sunroom is a great option, but if you don’t have one, place it near a sunny window, preferably a south-facing one. Be sure to place it near enough that it gets plenty of sun, but not so close to the window that the branches could touch it and get too cold. Avoid placing your citrus trees near vents or drafty areas like frequently opened doors, as drafts (hot or cold) can dry out your tree.
 If you don’t have a south-facing window or an area with enough natural sunlight, you can always add grow lights. How you add them will depend on your personal situation, but some options include clip-on lamps, adding a grow lightbulb to a floor lamp, or hanging grow light bars above your trees. Place the lights 12-18 inches away from the trees. Aim to give your trees 9-10 hours of light a day, whether that is all direct sunlight or a combination of sun and grow lights. Never give them more than 12 hours of light in a 24-hour period!

Citrus trees thrive in humid conditions, which is something most people struggle to provide indoors. Some ways you can create humidity for your citrus trees are to mist the leaves once or twice a day, place your pots on a shallow tray filled with pebbles and some water, or by adding a humidifier. There are many options out there to fit your budget, from simple non-electric stone humidifiers to electric humidifiers with built-in, adjustable humidistats that give you a live reading of the environment’s humidity.

Water when the soil is dry 2 inches deep. An easy way to check this is to stick your finger in the soil; if it is dry below your second knuckle, it is ready for some water. You want the soil to be moist but not soaked. Check regularly that your pot is draining well, as water built up in the pot can lead to root rot. If you notice the leaves of your citrus trees starting to curl, you may not be watering enough.

Put down the fertilizer! While fertilizing gives a necessary nutrient boost to citrus trees to help produce plenty of fruit, adding it during the winter can encourage growth. Winter growth can leave your tree vulnerable. Think of winter as a time for your trees to hibernate; saving their energy for the spring growing season. In the same vein, hold off on any pruning until spring.

Inspect your trees regularly. Outside pests can hitch a ride when bringing any plants indoors, so checking them regularly for signs of pests and disease will help you catch them before they turn into major problems. Wiping the leaves with a damp cloth once a week will help keep them free of dust and prevent infestations.

I hope this helps you succeed.