First Step is to Identify that your cactus has rot. Rot is a dark brownish-black spot that will start to ooze. Rot is caused by the skin of the cactus getting punctured some how, some ways are:
An insect or animal biting the plant
Debris in a monsoon storm hitting the plant
Someone purposely poking the plant
Another plant growing into the plant
Now that the rot has started, first keep an eye on it for a few days, sometimes it will stop spreading and heal itself. But if it spreads then you must cut it out before it gets too far and kills the plant.
When it is time for surgery, you will need a sharp knife, rubbing alcohol, paper towels, and a scoop or spoon of some sort. Start cutting out the bad spot like you would do on an apple. But every time you cut wipe the knife with the alcohol so you don’t spread the infection. Keep cutting or scooping out all the rot until you have clean flesh. Undamaged tissue is about the color and consistency of a cucumber.
Now that you have cut out the rot, it’s time to clean up the hole. First remember to rinse your tools with the alcohol every time you use them. Use the knife or scoop to clean up any jagged edges, then make sure the hole you’ve made tapers to the outside. This will ensure that any water or rain will drain out of the hole. The hole will always be a scar to the plant, so the cleaner you make it the less unsightly it will be.
After all the surgery is done you must sterlize. You will use bleach water, this is one part bleach to nine parts water. Now depending on the size of the hole will determine if you will use a spray bottle or a bucket. Hopefully a spray bottle will work, it is the easiest to work with. Now rinse the wound very well with the bleach water and let dry. If you have bleach water left, rinse your tools. Do not fill the hole with anything, leave open to dry. You have succeeded, congratulations.
-Be careful of the rot, it will stain anything it touches. The new clean hole you have made will turn brown as it oxidizes. This is normal. Just like taking a bite out of and apple then letting it sit out for a while.
-Be careful of the bleach, it can burn you and ruin your clothes. If you get it on your or your clothes, rinse thoroughly with lots of water.
-You can use these steps on all cacti, and most wounds or damage to your cacti, not just rot.