Purchase cactus potting soil, cactus food and a pot when you bring home your new flowering cactus from the garden store. Most cacti are sold in tiny pots so you should be fine with a 3-inch pot, but adjust accordingly for a larger cactus.
Loosely pack your new pot with potting soil, then remove your cactus from its pot, place it in the center of the new pot and mound fresh potting soil on the sides and top of the pot. If your flowering cactus is spiny, you can wrap a couple sheets of newspaper around the outside to grasp it without getting pricked.
Mix your solution of plant food and add this to the soil, making sure to note when it's time to feed again. Then water thoroughly.
Place your flowering cactus in a space where it will get lots of light. If you live in a temperate climate such as Texas or California, you can place your cactus outside and take the pot in during inclement weather.
Water the cactus when the soil is quite dry, testing the moisture content by sticking a pencil down into the soil. If there are still soil particles clinging to the pencil it's not time to water.