Make no mistake, recent R* games from GTA IV, EFLC, RDR (the predecessor) and GTA V are a walk in the park to reach 100% compared to this one. In addition, the guide identifies missions where you can also complete challenges much more easily than in free roam.
This is done combined collectables, challenges and other tasks all when you are in the right location, which reduces the amount of times you have to revisit the same spot. An unplanned approach to getting the 'Best in the West' Achievement/Trophy is likely to take well over 100 hours, so there are significant time savings from optimising the order you approach each requirement in, as outlined in the embedded video above. The key difference between this an many other guides on the internet is that it outlines the optimal order to complete every task required. The purpose of this guide is to set out a strategy to achieve 100% completion as efficiently as possible, with a linked video guide for every single task required for you to reach the goal (over 450 separate entries).