Acclimating Cacti from the mail
When you receive a cactus in the mail, assuming it was sent bare root as cacti should be sent, you should follow these steps.
Taking it out of the box
Care should be taken when unpacking plants from the box. Often times they are wrapped in some form of paper product as paper stays dry and breathes well. Tissue and newsprint are the most commonly used items. Special care should be taken with small and grafted cacti as they are generally physically more fragile than others.
Let it sit
Once unpacked, place the plant(s) in a warm spot that recieves indirect sunshine. You want them to be between 10-25C, try to avoid really warm or cold spots, such as a window. You also do not want to place them in bright sun because plants can get "sunburnt" quite easily when taken from extended darkness (such as 2 weeks in a box) to bright light.
Why let them sit? During shipment, and also from having the roots cleaned of all soil, some roots will definately get broken off. This leaves open wounds left unprotected. Even unwrapping the plant from a tissue can cause some unseen root damage, this is unavoidable, and not of great concern. The reason to let sit is to allow these roots to heal, or dry, before planting. Cacti are not like many other plants that like to be watered after transplanting. Cacti often prefer being dry, as this will prevent rot. Cut flesh and water are the 2 ingredients needed for rot. There are exeptions to this rule, such as Pereskiopsis sp.
Pot it up
After the plant has sat and dried for at least 2-3 days (up to a week is better), you may now plant the cactus into its pot. It is best to use *dry* cactus mix to avoid the water getting into broken areas right away. Because repotting also damages roots, generally the smaller ones, it is important not to water for at least 2 weeks after potting it up. One of the biggest mistakes novice cactus growers do is over water, or watering right after recieving the plant.
During this time it is best to avoid strong light. Because you are unaware of the plants growing conditions previous to your own, it is better to play safe. Slow and steady wins the race. Start off in a shaded area, and every couple weeks gently increase the amount/intensity of light. If you notice your plant looking faded or yellow, this is due to too much light. Also red colouration is a sign of too much light. We generally take 6-8 weeks to fully acclimate a plant to our greenhouse.
After the plant has been potted for at least 2 weeks, more is better, you can water. A good general cactus watering schedule in the growing season is water thoroughly so water comes out the bottom of the pot. Then don't water until the entire pot is bone dry for at least 2 days. This varies a little depending on species and climate.