When I build a frame (without a fixture/jig), I pin the lugs/tubes and then pull everything back apart for fluxing. Slather liberal amount of flux on everything and then drive the nails back in. Last step is checking alignment and tweaking as needed before brazing.
I do one joint at a time and make sure it’s perfect before adding each additional tube. This method is not ideal since heat causes distortion each time so I wind up doing unnecessary alignment bending this way. Despite this downside, this technique works well enough and the frame turns out straight. Needless to say, you need a surface plate to measure against. I made my own from a slab of steel with a pedestal in the edge which holds the frames bottom bracket – so the main frame tubes are parallel to the surface.
Good luck and hope this helps.