How you teach the rules of Go to beginners will depend on
Let's start by assuming the easiest case: this is your club's open evening, a good number of beginners have come along, they are intelligent adults, and you are prepared for them. Here is what we recommend:
The point of the first few games is to make the beginner comfortable with the rules and to give them a feel for the game. Remember that it is not essential to explain all of the rules straight away; in particular, details like ko and seki are likely to be confusing. Keep in mind at all times that it is hard to take in large amounts of information in one go. Because of this, you are likely to have more success if you describe just enough rules to get playing, then introduce further rules (e.g. ko) as necessary.
Begin by explaining the basic rules:
Try to be consistent with your terminology (e.g. always use the words "stone" and "turn" rather than "piece" or "move"), and avoid the use of jargon. You may find it useful to explain "liberty" and "atari", but that should be all.
The beginner now knows enough to start playing. You should explain the handicap system and give the beginner an appropriate number of handicap stones (as a rough guide, the beginner is about 30kyu and you should give 1 handicap stone for 5 grades difference on a 9×9 board). If you intend to let several beginners play amongst themselves, you can skip this detail at first.
This section assumes that you have arranged first games between pairs of beginners, with someone to oversee each game.
The first games should be played on a 9×9 board: this is less daunting for a beginner, and the game is over sooner so there is more chance that the players will see the consequences of their moves when the score is counted.
It is good to encourage beginners to play their first games quickly; many people in their first games think very carefully before making the wrong move! Some beginners can freeze completely, and need encouraging to make a move.
It is unlikely that either player will know when the game is over, and you should guide them in this. Don't just say "it's over now", say something like "I don't think you have any move that gains anything, if you can't see one either you may as well pass." Once both players have passed, ask them both their opinions of any dead groups, and don't proceed until until they have agreed. It is very off-putting to have a teacher remove your cherished group from the board for no reason.
At some stage, a ko may appear. At this point you should explain the ko rule – resist explaining its implications however! It is also possible that a snapback will arise; this often confuses beginners who often think it is disallowed because it is a ko or suicide, so you should take care to explain why it is legal.
Many beginners have difficulty understanding which groups are alive or dead at the end of the game. Often, they will try impossible invasions into your territory. Accept this: suggest to them that it won't work, and let them find out why it won't.
After one or two games, when you have captured several large groups, the beginner will probably be very keen to know how to avoid this! The easiest way to demonstrate eyes is to make a group with a three-space eye in the corner, and show what happens when either player plays on the vital point. You may choose to set up some simple life and death problems for the beginner to try to solve.
There are various things which are best done differently with younger children.
This is also known as "atari-Go". Use a 9×9 board. The rules are as for Go, except that the first player to make a capture wins – after which you stop playing and start another game. It may be necessary to remind them of this last clause, otherwise you will find them playing a game with no objective.
This is a kind of antidote to Capture-Go, to get beginners used to the idea of territory. The rules are:
If you have more than one beginner in the club, it is good for them to play amongst themselves. While the handicap system allows players of greatly differing strength to play, many people do prefer the chance to play against people of roughly equal strength. All club members should still, of course, be prepared to play teaching games against the new players, remembering that the more they do this, the sooner they will have more even game opponents!
Teaching games should be a constant feature of all go clubs; every member should be encouraged to give weaker players teaching games. This is beneficial for everyone; most people find that explaining a concept to someone improves their own understanding of that concept. Some points to bear in mind are:
Although Go programs do not yet challenge players above around 2 dan, they are very useful for beginners. It is said that beginners should lose their first 50 games as quickly as possible, and many people prefer to do this against a computer rather than against a human opponent! Playing a few games against a computer is a good way for a beginner to learn common shapes and simple tactics. Computer programs don't gloat when they win and don't moan when they lose, and most important, do not mind if you keep taking moves back. Some possible programs, all free, are:
The BGA publishes various booklets and leaflets  that may be useful. In particular, Andreas Fecke's Cartoon rules booklet  and the Play Go Booklet  are recommended as introductions to the rules of the game.