Matchmaking in Shuffle iT is, by design, pretty simple and versatile.
The Matching tab from the lobby can be used to start a 1v1 game, right now, with either a random person or a bot. The Quick Match button will pair you up with somebody else that is also looking for a match, without discrimination; the Good Match button will pair you up with somebody close to your rating. The Bot Match button sets you up with Lord Rattington, or whichever bot figures you to be worthwhile to play with.
Alternately, if you're looking to play with a specific person or group of people, you can create a table in the My Table tab, or find one in the Tables tab (wherein you can find the host of the table you're looking for, and click the options to play or spectate). In making a table, you can set the minimum and maximum players, as well as the set of cards to draw from to make the kingdom. By clicking Advanced Options, you can also change many other specifics of kingdom generation and gameplay. The game start when everyone in the table has clicked Ready.
You can edit these settings between games by clicking Edit Table between games.
If someone does not see an option to play in your table, try widening the range of available relative ratings; you can do this by clicking Advanced Options and then setting Max relative rating and Min relative rating to 100 and -100, respectively.