A casual shirt can be worn however you wish - to keep a laid back feel, leave the top two buttons undone and the collar somewhat splayed.
Men’s casual shirts should be ironed inside out, starting from the collar and button placket, before working out across the body and down the sleeves - however, try not to give too much of a defined crease, as they aren’t necessary for a casual event.
A casual shirt should fit well, but looser than that of a formal shirt to allow for free movement and comfort. Try making sure there’s at least a couple of finger’s width of freedom between your neck and your collar.
Casual shirts should be worn with casual outfits such as jeans or chinos.
Casual shirts generally sit to the line you would usually wear your jeans too - but not much further. They aren’t usually designed with the idea of being tucked into your trousers, so don’t require as much excess fabric.