First bike is a tubeset from
Cycles Nova..
It's 4130 bike specific butted tubing. I braze with an oxy/propane set-up. My oxy if via an oxygen concentrator (used medical unit). Torch is a little Uniwled 71 running specific tips for the propane (TEN tips).
This thread on Vsalon has great info on oxy/propane.
I used methods outlined in the Paterek manual to keep things true and aligned. Biggest thing is to cut accurate mitres. There is lots of reading to be done online, for your first few frames don't worry too much about how well aligned or accurate the final assembly is. A mm or two here and there will not have a drastic affect on how the bike rides.
A surface plate (old granite countertop is accurate enough) and V-blocks are way more valuable than bike specific jigging.
Build in sub assemblies. This is the bb to seat tube, aka the polo mallet. If your mitre is correct it just needs to weight to pull it keep the bb seated while you tack it.
Do the same thing for the head tube to the down tube.
This is how I brought the first two sub assys together into one. I used my two lengths of angle to hold it in the right spot to tack it. Again if your mitre is right this is all that is needed.
End up with this.
I used a strap to then snug up the front to hold the top tube in place. Another option would just be some steel spring clamps.
This is the hardest portion. You need to "float" the rear axle in it's position so that it will hold the dropouts in place to tack the chainstay to bb. I referenced my full scale drawing to give me reference points to position the lengths of simple L channel. It's all simple triangles to work it out.
Eventually you get this.
Then on it's very first ride you do some stress testing.