How to butt weld 2 pipes together straight?

attenuated said
I have used a piece of angle iron and c- clamps and then tack together.

HTH

Angle, I beam, or channel, anything of such dimension that the pipe will "lay" in

I used to "throw" sheet metal vent into a corner of the basement floor and the wall to get two pieces together straight.

I had a real smart college educated apprentice that couldn't figure that out.