Welding your own frame is an adventure ! Expect to make mistakes, but it makes you a better builder next time around. Maybe try straight gage cromo tubing or even high ten steel on your first projects. Move to the exotic stuff as you get better. As for jigs, I started out with wood jigs - just make sure to have a fire extinguisher nearby just in case. We visited a local town framebuilder in Italy maybe 30 years ago. All he had were hand tools, gas welder and a bike stand. No jigs. Everything was done by eye and simple hand tools. If you really want to do it, it can be done with some hard work and research. Your enthusiasm and ingenuity can trump the need for expensive tools.
Here's a frame I brazed over a decade ago using straight gage cromo tubing .035 wall thickness. Started out as a road bike but reviving it now to become a touring bike with more upright bars to match the aging rider. Changing it to a 1x drivetrain so this old geezer doesn't get confused as to which side to shift.
Good luck on your frame building adventure.
