You can actually get small bags of cactii compost, quite gritty, if that's easier for you to use.
Depending on the type of cactus I agree with Alina on position, but some cactii I dont actually water at all between Oct & March- still keeping them on the windowsill in my unheated NW facing porch.
However the 'Christmas/'Easter' types are woodland plants, so they do get mininum water over the winter & food, when I remember (!). All of mine are about to flower now. They are also in the porch & go outside, in a shadey spot, for the summer months.
Any idea of type/variety of yours? J.