By entering "who all o", you will see a list of all the players who are open for games. Players who are actively looking for games will be marked with an exclamation mark (!); you should try them first. Finally, you can restrict the list to a particular ranking range by typing "who 5k-10k o" to see the list of open players from 5 kyu to 10 kyu, for example.
After viewing the list of open players, pick one and use the "stats" command to check their information line. This way you can find out if your potential opponenet has any special preferences. Then use the "tell" command to ask if they want to play. If a game is possible, you should then agree on the terms of the game.
Continue to "rank".