A new player showed interest in joining the latest Timeshapers league but mentioned that they have no idea how to do so. I started explaining at which point I realized I might as well write it in my blog so it can be easily shared later.

Challonge
There are many online KeyForge leagues. Some use spreadsheets or private websites, but the easiest way to run a league is via Challonge. Challonge is not perfect, far from it. But for more purposes, it suffices. The platform allows creating and running tournaments in various structures such as Round Robin, Single and Double Elimination, and Swiss rounds. Once a tournament is created players can be added by the creator, or a public sign-up page can be made allowing people to simply sign up with a click of a button.
Leagues
Leagues are usually run as Round Robin, although Swiss and Elimination tournament structures are also in use. Leagues often require players to play one game a week and are a nice and relaxing way to enjoy organized play. Some leagues require decks to be submitted for the entire duration of the event, while others may allow changing up the decks from week to week. Challonge itself doesn’t distinguish between leagues and tournaments, so all events are called tournaments.
Tournaments
Tournaments are one-day events that most often use a Swiss structure with or without a top cut. Challonge does not allow Swiss tournaments into top cut to be automatically created, so organizers usually make a second Challonge tournament for the top cut. Top cuts are most often run as Single Elimination, and to qualify for top cut you either need a specific result (such as X-1 or better) or be in topX players at the end of the event.
Sign-up
There are two main methods used for singing up to Challonge. Either a private or public link will be posted to the Challonge sign-up page, in which case you can simply follow the link and click the big orange button to sign up.

In other cases organizers will use a google form for sign up. This is done because it allows organizers to collect more information such as Discord username, TCO username, and decks to be submitted for the event. Along with this information, they will likely ask for a Challonge username. It is important that the username will be exact, as the organizer will use it to invite the player to the tournament. You will get an email and a message on Challonge. Upon accepting the invitation you will be added to the tournament page the same way as if you signed up.
If you are signed into Challonge and have set your time zone in your profile, the tournament page will show the tournament start time in your local time zone. Very handy!

Playing
Once the tournament has started, the tournament page will show the bracket. If it is Round Robin or Elimination then the entire structure will be visible. While if it is Swiss, then only the first round will be shown. When you sign up the matches shows are a preview. Before clicking the start tournament button, organizers will (should) shuffle the participants to prevent matchups based on sign-up order.

It is best to wait for the organizer to tell you when the round or league have started before playing any games.
When playing, it is usually expected that the winning player will submit the result of the game. Clicking on the report score button will allow you to report the result.

Some tournaments use keys forged as tiebreakers, so make sure you know how you should report. In the case that keys forged is a tiebreaker and your opponent conceded before you forged your third key, you will still report 3 keys for the winner.
In some tournaments, the organizer will generate TCO links and attach them to the game. Those will show up as a blue paperclip when you hover over the report scores link. Before clicking the TCO link make sure all TCO windows are closed. Otherwise, TCO will show you are connected to the game when you aren’t. Your opponent will usually see you connected to the game and disconnected, so if you see your opponent disconnect from the start game window, let them know.
In tournaments without TCO links it is usually the responsibility of the top player to create a game. You can either simply wait for them to join, or better yet, send them a game link.

Games created for tournament usually have a uniform password to allow organizers to easily hop into the game in case judging is needed or time needs to be called.
Tiebreakers
Challonge allows for a number of tiebreakers to be used for tournaments and you can get a little more information about them by hovering over the header.

By default, Challonge uses wins over tied opponents first, which simply counts the number of wins over opponents with the same score. Second, it will use Median-Bucholtz, which is strength of schedule with the top and bottom scores eliminated. Other tiebreakers are points and point difference, which can be used when results are reported as things other than simply win or loss. For example, keys forged.
Armed with this information you can go ahead and find some leagues to join. For the Timeshapers league you can join the Sanctumonius-Timeshapers discord. I try to run one every couple of months, so be sure to stick around if one is not currently open for sign-ups.