Professional (not me) bicycle frame builders have very sophisticated table, jigs and tools to ensure their frame is perfectly aligned. Me, well, I have some plywood.

A table with pivots was built, with adjustable height

And then covered with white paper. I drew lines for the major build points, and checked everything for square and level.

When I go to sensitive alignment elements, I added 4 legs to ensure the table was level so that a level could be used on frame elements.