Cacti grow in desert-like environments that can be found in regions all over the world. Desert conditions vary from region to region, but all share common characteristics that are important to remember when trying to grow cacti indoors. Deserts are defined as regional geographic areas that have significant wait times between rainfall, low air humidity, high winds, and high daytime temperatures.
The most common mistake made by cactus growers is insufficient sunlight. They require 5 hours of direct sunlight a day. This amount of sun will not burn them, and cacti even grow thorns to provide shade and protect their internal water supply. If you do cannot give them direct sunlight, grow them as close to a window as possible, or supplement with artificial lighting.
In nature, deserts can go months at a time before it rains, and then experience flash flooding. This is what you are trying to recreate at home.
The soil must be perfectly dry all the way through before watering it again. When you do water your cactus, fully saturate the soil. Do not spray or put a few water drops on your cactus, it will need a good full watering in order to rehydrate and store more water and grow. Under-watering your cactus can be just as dangerous as overwatering, so do not be afraid to give it a good drink when it needs one.
Since cacti are such slow growing plants, they absorb very little nutrients from the soil. Because of this, it is not necessary to feed cacti, especially if you use a good potting soil. If you do fertilize, use only cactus fertilizer, or diluted solutions of all purpose fertilizer on your plants. Be sure to water with clear water at least twice before feeding again. Do not feed cacti during winter months, as they go dormant and slow or cease growing all together.