You don't need to log in anywhere. You simply play a game against someone else who's already joined the Ladder site, too. The loser of that game will post the score on the Ladder site, and if you managed to beat a player ranked higher than you, that will move you up in rankings.
Particularly interesting are 4-somes, because these can include as many as 18 Ladder games (!) rolled into a single 18-hole round. That's because everyone plays against everyone else on the Front 9 & on the Back 9 for two separate Ladder 9 games, and everyone at the same time also plays everyone else in a Ladder 18 game. Practically every shot counts. If you bungle the front 9, you can still win the second Ladder 9 game on the back 9. Of course, you must count with up to 3.5 hours of play for such an 18-hole foursome.
If you need more assistance, feel free to ask. You might also want to read the Ladder site manual.