Tuck, I went through the same thing a while back. I tried and tried and tried and couldn't get it. If it hadn't been for the folks here, their help, advice and support, I'd have given up. I got the hang of it at last though.
Bill just covered the basics. Two things that are very important though are:
After you grind both surfaces clean, very clean, flux it well.
Put the sandwitch together and either tack weld, wire or somehow hold it all in place.
Bring it up to temp and smack the He!! out of it to try to seal it right off. That's seal not weld. This is important:
It will take you several heats to get a good weld. Don't expect to get it the first heat, just shoot for it to stick the first time and weld it on the next few heatings.
A rock solid anvil is a MUST for forge welding.
Good luck!!!!