The below is based on my own Cactus experiences.
The water requirements of a cactus are one of the trickiest things about them; It’s said to be far easier to kill them by overwatering rather than under watering. I use rainwater when I can.
Spring- Autumn: Water but always leave to complete dry out I usually water my boys once every two weeks.
Summer: I water them once a week but again allowing the soil to completely dry out first, which obviously happens a lot quicker in the summer.
The first thing you must get right is their soil, I mix soil with sharp sand, fine powder sand should be avoided as the reason for the sand is to keep the soil open and create air and the fine stuff wont do that. I also put gravel at the bottom of my pots just to create extra drainage. But your local garden Centre, Homebase or B&Q do specialised cacti soil so if you haven’t got the time just go with that.
I try and give my Cacti the most sun possible so a Window sill is the best place in our house. I also rotate my cacti so that each side gets the best of the sun.
A clay flower pot is best and always buy a pot with a drainage hole. I read somewhere that clay pots act as a store for water and enable the roots to breathe more easily. If you are going to re-pot your cacti best to do it in the spring but bear in mind cacti love to be cosy in their pot.
An awesome tip if you are unsure weather to re-pot with most of the smaller fat boys if he has almost reached the sides of the pot it is time to re-pot. Leave about 1-2cm space between the cacti and the outside of the pot. Always try and remove as much of the old soil as possible before repotting. (See below)