practice. i use to be really into casting and got pretty damn good at it (i could go full cpt flowers mode in a big teamfight) but have fallen out of love with the game for the most part since i have been playing for almost 9 years. personally i would start by casting over lcs games with the actual casters in the background, helps with the flow and pacing of the game and your casting and what you are focusing on.
there are also two types of casting, color casting and play by play casting. color casters are the more statistic/macro/micro casters that pay attention to the smaller details while play by play is team fighting and skirmishes and what not. if you are in a multi caster game, one focus on one of the sides while the other does the other side. ie flowers is a play by play while jatt/kobe is a color. flowers got found by casting collegiate games
if you get really good and show that you are really committed, one of the lcs casters may help you and "coach" you if you reach out to them. most of the casting they do is off the air while practicing