Don't run out of batteries at a wedding reception or other event as your flash struggles to keep up with your ISO speed. The idea is that if you know your photos won't ever get printed larger than 8x12 and that it is more likely that they will never exceed 6x4, even in an album, you don't need a low ISO setting. The other consideration is that this takes place indoors, in a moderate sized place (no bigger than a hall) and works better with lots of walls to bounce off of. So here's the meat of the settings:

ISO 800-2000 (usually 1600 works well)
f/5.6-8 (5.6 is better)
1/45th shutter speed (Allows ambient light fill for warm color
External same brand as camera flash with TTL metering of some kind
Use basic plastic diffuser on end of flash
Attach flash with same camera brand off shoe cable
Hold the flash above head, usually to the left, though exact direction does not matter as much

Another useful tip if you find it difficult to steady the camera holding both it and a flash then, for example, tilt your body right and rest the camera on your chest or left shoulder, holding the flash in your left hand, making a right angle with your left arm. That way you get a flat surface for your camera and elevation for your flash.