Actually, as I think back, the billet was 4" long, not six.
O-1/1084 in 1/4" is a simple and affordable starter mix. The combo will get a good contrast and work easily. It will weld up nicely.
15N20 and 1084 is a good damascus mix, for sure, but the cost will be higher, and the layers will be unequal.
Starting to learn forge welding with even thickness layers makes the process simpler.
The stack will be 2" wide, which is pretty easy to weld up. 1.75" tall, which is shorter than the width, so it will resist tipping sideways as you forge it down. And, 4" long, which can be done in two or three heats by hand.
Here is a basic technique I have used for a simple billet:
Cut two bars of O-1 2X4X1/4", and two bars 2X6X1/4". Cut one bar of 1084 2X24X1/4", and two bars 2X4X1/4"
Stack O-1(4") - 1084 (4") - O-1(6") - 1084(24") - O-1(6") - 1084(4") - O-1(4"). The longer bars will be the handle.
General Notes: Once stared, keep the billet above red heat. If it cols off before the first weld is set, the billet will never weld up right.
Every time you pull the billet out of the forge, brush off with a steel wire brush. Flying flux is not fun. It can burn you and start fires twenty feet away from the anvil. Wear full protective gear and keep the kids away when welding.
Each time you put the billet back in the forge, brush the scale off the anvil face, and place your hammer where you can pick it up easily and quickly. Work fast, but not in a hurried manner. Smooth and even forging is the trick.
OK, let's make damascus.
Grind off all mill scale and clean all bars well.
Clamp the billet in a vise, and weld all four corners ( see note on bottom). Then weld around all the handle seams
Put the billet in the forge and heat to red hot.
Remove and flux sides well, and continue heating.
As the bar reaches orange-yellow heat, brush off with a steel brush, and re-flux.
As the flux gets runny and looks like hot butter, brush again and re-flux. The billet should be approaching yellow heat by now. Try and hold at this temp for a few minutes to allow the center of the steel to get as hot as the outside. It all has to be at welding heat all the way through.
When the billet gets hot enough that a piece of coat hanger wire will stick to it when pushed against it in the forge - It is ready to set the weld. Note- If the billet starts sparking, it is too hot.
Pull it out, brush it off quickly, put back for a few moments, and then set the first weld. Go down the billet from the handle end toward the other end with firm BUT NOT HARD blows. Work the billet from the center toward the edges, and cover the whole billet. Flip over as you go, working from both sides. If it starts cooling off, re-flux and re-heat. Take as many heats as needed to get the whole billet solid.
You should feel the weld take with a soft "Thud". Brush off, re-flux, re-heat, brush off, re-weld again. Repeat a third time ( or as many as needed) , using heavier blows. The billet should now be feeling firmer, and acting a bit more like a solid block of steel. You will notice that the seams on the sides are staring to become all one red color, and loosing the dark line between them.
Brush off well, re-flux, and re-heat.
Pull the billet and brush off the sides well, re-heat for a moment, and weld down the sides. If any of the joints are not welded, they will show up easily. Work the billet into a solid rectangle of steel. If all is good, you are ready to draw the billet. If there is an open joint, re-flux, and re-weld until all is solid.
Once the billet is a solid block of steel, start drawing the billet with a cross peen and work from the end toward the handle. Stretch the billet until it is twice the length. The billet is drawn at temperatures from orange-yellow heat down to red. Re-heat whenever the billet gets to medium red. Once the billet is about 8-10" long you have a decision to make.
There are two methods of folding a billet by hand.
One is to cool the billet, grind the face clean, cut in half, weld the ends shut, and then repeat the welding and drawing process.
The other is to never let the billet get below red heat and do all work hot. I use this method, as taught me by Jim Batson. I call this "Hot Welding".
To do a Hot Weld, when the billet is drawn out long enough to fold, brush off well, and stick back in the forge. Drop a hot cut in the hardie, and pull the billet out. Brush off and then hot cut in the center nearly through. Brush off, re-heat a bit, flux, brush off and re-flux lightly, and fold over taping it snug, but not hammering on it. Bring up to welding heat, brush and flux again, re-heat, flux, weld, etc. On each weld up, make sure to work the sides,too. If the billet isn't solid , keep working it before drawing, or the seams will split later. When drawing the billet, work evenly and in 3-4" sections per heat.
Every time the billet gets long enough, just repeat the folding process. Make the folds up or down, so the billet stays balanced on the handle. In about an hour or so, you should have made five or six folds. Once the final layer count is reached, draw the billet out to about 1/2" thick. take a break, and let it cool off.
Cut the handle piece off, Grind all surfaces clean and inspect for bad welds. If there is a problem, try and fix it by re-welding that area. If it won't re-weld, try grinding it out. You will be wasting time to try and forge a knife out of a billet with a bad weld in it.
At this point the billet should look like a bar of steel, and show little or no layers. Etch it in FC for a minute to see how the layers look on the side. Rinse and neutralize well. If everything is good, forge the bar out to whatever size you want - 1/4" to 3/16" by 1.5" is the norm. You should have enough for two knives.
Footnote:
A good technique, once you have done a billet or two, is to weld all seams of the billet shut. This is called dry welding. There are several threads about the technique. The use of kerosene in doing this eliminates flying flux almost completely. Learn how to weld first, then explore this .
. I just picked up an empty R-134a bottle, so I have my forge body. If I can get it all together, I'm going to start slowly collecting all the parts needed for a small propane forge. 
