Christmas cactus are an epiphytes (grow in trees) so, while they are a type of cactus, they don’t grow in a sand, desert type soil.  Regular potting soil or even orchid soil would be more appropriate.  Simply break odd some sections, let them air dry for an hour  and slip them into the soil.  Lightly moisten the soil.  Don’t keep the soil wet all the time.  Wet it well then let it get dry between wettings. Christmas Cacti root very easily.