In reply to jamesofdeath: There are numerous methods but the one that both Plas y Brenin and JSMTC teach, and I've seen many other instructors use is as follows:
Measure appropriate distance between climbers.
Take approximately a 1metre bight (rope dependent) and tie overhand at point measured out.
Now tie 2nd person in as follows:
Tie an overhand on the bight about 30cm from the first overhand to provide some leeway for 2nd climber.
Thread bight around harness as normal.
Feed bight through the overhand knot.
Tie a single stopper knot.
Clip remaining small loop back to the tie-in loop you have formed with a small screwgate.
The MLTUK Handbook - Rock Climbing, Essential Skills and Techniques by Libby Peters (page 187 of 1st Ed.) describes a very similar method but re-threads the overhand rather than passing the bight through and tying the single stopper knot.
Either method works but with the variation I described it is much easier and quicker to get a well-dressed knot in a thick single rope.
Hope that makes sense.