Download a software called MacroGamer by searching for it. You will have to learn the software yourself, it's a bit difficult to explain here.
Once you have a Macro set up to unbolt bolts on any part, you can leave it AFK to do its job. The best recommendation is to use the I4 engine as the 'Engine Head (I4 C)' has 5 bolts inline and it can save some time. Just remember, it's 500,000 bolts and each unscrewing will take half a second at least, plus the time to select and unselect the part. If you use the 5 bolts in a row of the I4 engine, a rough calculation of every cycle at 5 seconds puts the total AFK time a bit under 140 hours. It will be worth the time to optimise your macro to reduce the time taken per cycle.
This is gonna take a lot of days (plus the game tends to crash on occasion). I also recommend keeping the game at the lowest graphic setting and window resolution. Plus make sure your inventory is almost empty and you have no cars in the garage (parking is fine). This will ensure your computer generates the least possible heat. It will make the game crash less and throttle performance less too.
PS. Combine this achievement with my 'Connoisseur (Gold)' guide for efficiency.
Good Luck