Hi Folks
I trained in mechanical engineering and spent ten years in the steel shipbuilding industry, where I designed everything needed to make a ship go, from anchor winches to propeller shafts and everything in between. I thought I knew a lot about machining stuff from designing the parts but experience then showed me that I only knew what the part was supposed to look like - getting from metal blank to finished part was a whole 'nother ball game. Okay, I was probably a bit better off than many, because I already understood limits and fits, tooling limitations, choices of materials and the issues involved with machining them, but my first attempts to machine stuff were abysmal failures and I had to go buy a bigger scrap bin for all the metal I was throwing away. I learned the hard way about taking the time to properly dial-indicate a part for alignment on my long-suffering Sherline mill. I learned the hard way that there is no such thing as a cheap drill bit, and using indexable ceramic cutting tips on my lathe for the first time was like climbing out of a Model T and climbing into a Shelby.
Everything you do has its own unique challenges, so you are constantly learning, correcting, refining. I advise anyone wanting to try machining to start simple, read books, bug experienced machinists, ask questions on this forum. Harold Hall is one author of many books on workshop subjects. A statement of his made a big difference to the quality of my work; it went something along the lines of "It will often take more effort to build a fixture to hold the workpiece, than it will take to make the part itself".
Don't skimp on measuring tools either - my success/accuracy on my mill improved a lot when I bought an edge-finder tool - before that, I was constantly introducing little errors through shoddy set-up despite spending an awful lot of time on it. Having been raised in a metric country, I found it impossible to do anything well on my 1960's-vintage lathe so I bought digital read-outs and bolted them on - quite apart from the metric aspect, being zero-resettable was a very useful feature in its own right, and so I bought another set of DRO's to bolt onto my mill!
A very important thing to remember is, you can't expect to get it right from the get-go, a lot comes down to experience and even very experienced machinists stuff things up now and then, so don't be afraid to dive in.