Doing flash exposures manually is actually dead easy once you get the idea, but really intimidating (though I'll ignore fill in flash for now).
Originally Posted by badoix
Every flash has a number - the guide number. A small flash will be about 20, while a bigger flash might go up to 45 or beyound. 20 is fine to begin with - there's no point in carrying round a huge heavy flash when you don't need it (though of course more powerfull is always more fun: "call that a flash... that's a flash!"... repeat in Ozzie accent while waving metz around).
though most flashes have an automatic setting, I'd ignore it unless it's a seriously nice flash, and set the whole thing on manual. You can pick up a GN20 flash for next to nothing, and it will work fine, provided you don't rely on the meter.
Assuming 100 film, just estimate the distance to your subject in metres. Divide the guide number by that distance and you get your f/stop.
eg: 5 metres away
so set on f/4.
If you're using 400 film you get to double the guide number for your flash.
So the same calculation would be
Shutter speed is irrelevant (provided you're slower than the camera's flash sync speed - probably marked with an X) unless you doing fill in flash, which I'll leave to another time.
I explain this more slowly on my website (along with some pictures which prove that it works!) - see the link below.