John,
Here is how I was taught and it works pretty well. It assumes you are using coal. Make a beehive style fire. Use a deep and high fire, damp coal, forms a hard shelled dome. In the front of the dome facing you open the dome enough to put you pieces in and still see the part in the fire. A slow constant air flow works best. The inside of the fire should look yellow.
Now look closer and you will see three shades of yellow, this is important later. Get the pieces to an orange, brush off the scale and put on the flux and get back in the fire. Watch the pieces. For larger stock I turn them a bit to get an even heat. When the steel's color matches the middle of the three shades of yellow and looks oily its welding time. When you pull the steel out of the fire it will smoke like mad. That's the boraxo(I use it too). I hit in the center of the length to be welded and work my way to the ends, trying to force the flux out of the weld.
I hit light as I found hitting hard breaks the weld apart if it get cool. I repeat the whole operation, flux, heat and hammer until I am happy with the weld. I was told that for the first several heats after the weld is done heat to the same temperature to work the piece.
Others here have way more experience and better technique. Hope it helps.