Tools are essential to human life: that should be plain to anyone who has eyes to see. When it comes to achieving equilibrium, it is important to have access to a wide variety of practical tools, the ability to use those tools competently, and the ability to create those tools with consistent precision. The principle of Self-sufficiency/Decentralization dictates that any self-sufficient community ought to have an abundance of people with these abilities, for this is more efficient than relying on tools provided by outsiders.
The foundation of any civilization capable of precise construction (and perhaps any civilization, period) is a stable, perfectly flat surface. This flat surface is used as a reference to produce other flat surfaces, which serve as tools for construction. To be more specific, in human civilization, a stable, perfectly flat reference surface takes the form of a granite surface plate. This is a rectangular slab of granite which is usually at least 3-4 inches thick. The thicker the granite slab is, the more resistant it is to damage, and therefore the more stable it is. What turns any old granite slab into a granite surface plate is a process called lapping. The aim of lapping is to slowly and carefully remove a relatively small amount of material, in order to produce a consistent surface. Lapping can be used on more than just flat surfaces; but, we are indeed concerned with flat surface here. The lapping process is guided by measuring tools such as autocollimators (and lasers, in modern times, though the use of lasers will not be advocated for much here).
It is impossible for humans to produce a perfectly flat surface. In fact, there may be nothing in the 3rd dimension (also known as "the physical universe") which is perfectly flat. Still, we can get close enough to perfection in order to fulfill our needs on Earth. There are varying grades of flatness: a grade which suffices as a reference for one tool, may be too imperfect of a reference for another tool. When it comes to granite surface plates, the grades are B, A, and AA, with grade AA being the flattest and of the highest quality.
Granite surface plates are used as a flat reference surface to create other flat surfaces, the most noteworthy of which are measuring tools. Measuring tools, by definition, are used as references for making other objects. Straightedges made with reference to a granite surface plate can be used as references for forming flat surfaces on machine tools (mills, lathes, surface grinders, etc.). The machine tools, in turn, can be used to produce other tools. The closer to perfectly flat the machine tool's flat surfaces are, the higher the potential level of precision the machine tool is capable of ("potential," because there are other factors in a machine tool's capabilities, also).
A more in-depth discussion of precision would be better left to the Precision page. The remainder of this page should be dedicated to discussion of how to make tools.
Note: You may think the title "Useful Tools" is redundant. Tools are supposed to be useful by definition. However, especially with modernity, there may be tools which are excessive, sub-optimal, or even entirely unnecessary. To state it one way, you do not necessarily want to spend your precious time learning how to make a cheap, fragile, plastic version of a tool, when you could instead spend your time learning how to make a durable, accurate, metal version of the same tool. In order to reinforce this point, the title "Useful Tools" will remain as-is.