Before drawing on the tubes, I needed to figure out which way they bent, since it was pretty unlikely that they'd be perfectly straight. To do that I used an alignment table. An alignment table is a big slab of stone or metal that is ground carefully until it's perfectly flat across the top. It seems like any table would work, but it has to be pretty damned precise to make this work. I rolled the tubes on the table, looking at the gaps underneath, until I found the flattest axis... then marked those as the "sides" of the tubes. The center-line would be 90 degrees around the tubes from the "sides", because the tubes being slightly curved towards the front or back of the bike wouldn't make as much of a difference as having them lean slightly to the right or left.
Actually making that line is one of those things that seems easy, but ends up being really hard. Lining up a straight edge with the tube is difficult. Keeping it there while you mark things is pretty impossible without designing some kind of a fixture to hold everything. I basically held two tubes together, with the point that I wanted the lines to start from deep in the "V" between them and dragged a marker down the center... again and again until the line started where I wanted it to.
I then used the alignment table to check if they were straight with a specially made caliper. If both ends of the line were the same distance off of the table, then I was good. Otherwise, back to the drawingboard.
When finished, I taped over them so that I wouldn't accidentally rub them off when working with the tubes. EDIT: NEVER DO THIS. Taking off the tape is a full time job. Next time I do this, I'll just get the miter templates and bottle cage mounts aligned right away.
That's it for today, next time I'll share the diagram that I'll be working off of.