How does one become a Caster?

You would really need 4+ years of college or a history of casting to get Riot's attention. I think at the moment Riot has like 15-20 Casters at the moment so I think they are fine in the casting department but I'm sure if you can make a name for yourself they can make room for you.

If you have passion to becoming a shoutcaster for Riot I would start a Youtube channel and cast some LCS games with Play by Play and give your 2 cents on how blue side can win the game or red side vice versa. You will also build a rep for yourself and get your foot in the door.

Some casters played in the pro scene like Deficio,Jatt and Kobbe. Ninjas-Dignitas-CLG.
Some casters are more around the humor side of League to make things less stale like Phreak. Others are more for their play by play side of things like Rivington.

Pick a game to cast about I would recommend Team Curse vs Complexity back in 2014. I'm recommending that game because it's about 75 mins long and it's good practice for you. Being it's a long game you will need to keep your "fans/audience" entertained for the full game while keeping up with your play by play for the full game.
Anyone can cast a game that is only 20 min long but can you do it for a game that is over a hour long ?

Work on your play by play casting, this should be the easiest for you if you already have an "encyclopedic knowledge" of champions and their abilities. Similar to Rivington.

Work on your pick ban part of the game. After a ban is made pause the game and talk about why X team banned that champion and same thing with certain champions picked for a certain team. This will help you with an idea for what the specific team will be going for. For example if blue team picks Jax that is a hint for you to talk about how blue team will be going for a more split pushing strategy etc. Similar to Deficio.

While a game is in a stalemate situation you can talk about how each team can win based of different situations and strategies. Similar to Jatt.

Last but not least be funny and give a fresh outlook on things. Sure you can cast but if it's cut and dry people will get bored so pull a few jokes once in a while but make it League related last thing you want is to pull a "race" joke or a mama joke. Make it League related or not offensive and you should be fine. Similar to Phreak.

Start a Youtube channel, build followers, make a name for YOURSELF, and Good Luck. Laters

You can know the ability of every single Champion, but you'll never make it if you have no sense of how the game is play.

Casters in LCS don't just talk about Champion's ability, they're also consider as "Analyst" because they know and talk about the game as an LCS level, not Solo q level.

If you don't understand how a game should be play in Solo q you would never make it to LCS as a Caster.

You would need to know and talk about how a Team could of done this better if they did this, and predict how a Team is going to do this or this that will lead them to victory.

If you make a dumbass "Comment" as a Caster that's bronze or anything......you have no chance

Casters in LCS, there not just Caster for no reason.

Some of them are former Pro Player since S1 They're good at the game and are high Ranked too as much as to Plat 3+ and aren't Bronze

First of all you have to get started by casting local games and little to no important tournaments. If you are good, you will get noticed, cast bigger events and ultimately you can go to Riot like "Hey, I have been casting for this long, here are some YT links of the games I casted before, gimme job."

Maybe start your own YT channel and cast games of your friends, keep an eye out for those small online tournaments, contact the organizers and make a deal with them that you will cast their game on Twitch, LIVE then post the VOD on your channel.

Post videos about patch rundowns, META reports, PRO scene and get known as someone who knows what's going on. Joining some podcast may do you some good as well so you can prove that you have mic experience.

That's what I would do to get into the casting.

Ya honestly just do commentarys on peoples games as practice. Maybe even post them to youtube.

Without knowing if it works, I'd say, start getting a following by making and publishing commentaries. A Major in English can get you a job in Germany as something like a caster, but the best way to get recognized as one is having a following for your actual casting. I'm not saying it's easy, but it's probably the way to do it. Oh, and important, don't commentate your own games. This'll give you more of a lets-player kind of commentater and for sure won't land you as a caster. If you want to be a caster, act like a caster. Become a personality that people want to have as a caster.

Ask a youtube streamer/LCS player if you can use a recording of them (probably a more pro level streamer like Redmercy or Nightblue3) and cast to it and maybe submit a few to Riot and get them to notice you :) maybe email Dash or someone who is already a caster and ask them for opinions on how to improve and how to get noticed.

Honestly, at this point, getting this kind of job is all about connections.