I used two mountain bike frames; the rear triangle of one and the down tube with bottom bracket still attached and some of the seat tube for the front derailleur. (At least 4.75 inches) Set the head tube angle with plywood cut to the right angle or use a protractor square.
I was going to use the cut head tube as a guide, but decided that I needed a steeper steer axis for less trail without adding more rake. I like a small amount of trail (1.5" to 1.7") for slow speed hill climbing, because more than two inches will create too much wheel flop!
Grinding and hand filing the compound joints can be a challenge. A Tube Notcher jig would be much easier. But if you’re doing just one bike it's not worth the money.
If your using old rusty tubes, wash them out with Ospho rust killer.
Add heavy duty rear disc drop outs after the frame is done or at least tack welded.
Check alignment with string.
If you’re going to pay someone to do the welding, find someone experienced with a MIG welder and that knows how to keep the frame from warping. i.e.: cool each joint before going to the next…. Do the top of one side , then the bottom of the other side, then bottom of the first and finally top of the second side. Don’t do the sides until all the tops and bottoms are finished.
Some people say that TIG welding is much better, but it is very slow and will cost you much more. Fillet brazing is good if you’re going to do it yourself.