Euphorbia meloformis

I've had this guy for about a year or maybe a little more, and I've watered her a good five times. I'm scared of underwatering, as I just killed a mermaid tail cactus due to it. Is there any tips you guys can give me regarding watering? Should I water when the soils dry, should I wait a few more days after its fully dry? Etc. any help is appreciated!