If your curious - get a book called "the foundations of Mechanical Accuracy" - tells you exactly how to do it - and the problem with molten tin etc is the actually have things like a meniscus etc - they will gave you a starting point - ditto water and a plumb bob

The short answer is that by taking THREE squares that are basically flat, and touching one to the other, a to b, b to c, c to a and removing the high spots on each in order, rotating 90 degs between sets (the above was one set) will allow you to make a set of plates that is arbitrally flat

The thread is basically - gouge out the best thread you can - say on a wood lathe - but on a metal rod. Cast a lap around part of the rod, say, 1/8th of the rod - lap the whole rod in - it evens the errors out. Now use your rod to create a NEW rod, the way a metal lathe does (rotation of the work also turns the screw, which drives the cutter). repeat this process of averaging the errors by using long laps and cutting new threads. When your done, you will have a threaded rod that is NOT the original spec'd thread, but some multiple of it. Repeat, but with the gear train adjusted to give you the treads/inch you originally wanted...