I do as Stacy indicated, with a couple of exceptions. I do not grind off the mill scale, and if you do, I would encourage you to ensure the grind marks run side to side and NOT lengthwise. Secondly I usually discourage welding the end of facing away from the handle. Doing this will often produce inclusions because the billet expands, where as the tack weld doesn't, and the individual layers will sometimes "bow". If you couple this with lengthwise grinding marks on the materials, its almost a sure bet that you'll get inclusions.
I also discourage folks from hot cutting billets and folding them....draw the billet out, grind both faces clean (I use a 7" angle grinder), then cut the billet into equal lengths and re-stack it. Again, with the grind marks running side to side, with a slight crown (scarf).
Materials wise, I use 1/8" 1080, and .060-.070" 15N20. The reason is: Because of the nickel content, 15N20 does not compress as readily as 1080, and I like to have the finished pattern come out with as equal thickness layers as possible (unless I desire something different). Generally with the sizes I indicated, at around 300 layers, everything comes out looking equal thickness.
