Without leaping into the bells and whistles of all of the various UI programs and the engines, I am going to give you a simple example of how to play chess against one of the engines, first using the xboard interface and then against the scid interface.
I did say a couple of posts ago that xboard is a little clunky, and that does include starting it up. I am going to give a quick example of playing xboard against the stockfish engine, which you will find will give you more than enough of a challenge at first.
First, you need to open up a terminal window. This is because starting a UCI engine in xboard needs to be done using command line switches, although if you do this often you will want to create a script or polyglot config file to achieve it.
Fortunately, the command line isn't that complicated in recent versions of xboard. As mentioned, open a terminal (or konsole) window and type this command:
xboard -fcp stockfish -fUCI
Fortunately, xboard understands the "-fUCI" part of the above command to mean that the engine is a UCI engine, and although xboard can't understand UCI commands, it knows well enough to start polyglot to do the conversion for it. You can now select the option "Machine White" or "Machine Black" on the Mode menu, and have a challenging if uninteristing game (engines are good, but they aren't imaginitive, if you know what I mean).
In fact, the following command is quite interesting too:
xboard -fcp stockfish -fUCI -scp glaurung -sUCI
... then select "Two Machines" from the mode menu and watch stockfish play against glaurung.
scid is a different beastie, because it's primarily designed for analysing games and positions rather than playing. If you want a game of chess against stockfish using scid you can do it this way:
Firstly, start scid from the menu or by typing "scid" in a command window.
On the Tools menu you will see two menu items, Analysis Engine and Analysis Engine #2. Pull down either of these and a list of engines will appear. You can add more engines to the list by clicking "New" and typing the engine name and command, and leaving most of the other settings as they are (you need to select the "UCI" button to say whether the engine is a UCI engine or not).
So a quick summary of the engines I have loaded, which is the same as the list that you'll find loaded if you installed all of the packages listed on this page: http://blog.chess.com/delatbabel/ubuntu-linux-and-chess----installation
||ELO rating (approx)
Note that you don't have to load all of the engines mentioned above, but loading some UCI engines is better for analysis. Also scid will only allow you to play human vs computer games against a UCI engine.
Once you have your engine(s) loaded, you can select "Serious Game" from the Play menu and you're away. You can select the engine from the list on the play screen, and even set an opening if you want to practice your Nimzo Indian or Evans Gambit or similar.