If at all possible, get somebody show you how to forge weld.
Ten thousand pages of descriptions aren't worth seeing it done once.
For what is worth... steel should be fluxed in borax or silicaceous (or how the hell you spell that) sand. The latter being far better (tried both).
Do it at a red heat.
The pieces should be DAMN CLEAN on the surfaces that must weld. Grind them clean, don't touch them. Yes, grease from fingerprints will be annihilated by the forge blast, but meanwhile it'll do things (bad things) to the steel.
Wait until the flux bubbles, the forge inside is white and too bright to watch without making your eyes water (which is a good suggestion your body gives you that you SHOULDN'T watch in the forge at such heat without proper eye protection! At such heat the forge will project a lot of IR and even UV that your eye can't see... but it will damage your eyesight!).
The steel itself should look as butter about to melt, except butter (usually) doesn't shine like a nuclear reactor gone bad...
White bright is the key word. Local illumination can vary a lot what the steel will look like. Welding is better done in a subdued light.
If the steel looks like it's too cold to weld, than it is.
Don't get it sparking, though, as it means you are burning it. And a hammer blow will probably crumble it like old clay.
You should get to welding temp with a neutral atmosphere. In a coal forge with proper refractory materials, you can even do that in a reducing atmosphere, and you won't need flux at all (I know people who forge weld this way without flux at all!).
The fire has to be deep and clean, I can't stress this enough, and the piece must be kept out of the direct blast from the tuyere, if possible.
If you use a gas forge, the burner must be properly tuned.
When the weld takes, the hammer will feel deader when hitting the metal. Otherwise, everything will have a more springy feel.
Once you tried it once and succeeded, you will know what I mean.
This said, I repeat: go and see it done, and have somebody qualified teach you do it.
If at all possible, do it at your own forge, so he will be able to point out any problems it may have, and you will learn to do it in the conditions you usually work in.
