Why plant Cacti? A) You just cant kill the things B) Too easy to grow them C) Needs only about half a gallon to a gallon of water a month or so.
Step 1: What you need!
-Cacti (No breaking branches of living cacti plants!)
-Potting soil (No specialty varieties, Either all purpose soil or Soil designed for Cacti)
-Root growth hormone powder (available at Lowe's)
-What you want to put with the cacti(Rocks, Broken terracotta, etc)
-Lots of sun, I mean all day sun
Step 2: Planting the cacti
Wet down the soil just enough to make it easy to mold and so that it will activate the Root Growth hormone.
Dig a big enough hole to support the cacti and then put a small amount of the Root growth hormone(a lag screw helps with making the hole).
Then put your gloves on and put your cacti in the hole and compact the soil around it (Not too hard!)
Step 3: Optional: Add your rocks and things
Now go ahead and add the rocks and terracotta pot or what ever you have.
Step 4: Watering
I go out every week or two and give the cacti just a little spritz of water and thats it...and I live in the desert so you shouldn't give them a whole lot. The only time you should give them a decent amount of water is when you are getting them established.