100% Completion is achieved when a set of designated tasks has been completed. The following list provides the specific requirements for achieving 100% completion. This accomplishment applies to the main game - the Undead Nightmare DLC has a separate 100% completion.

The links below, details everything that needs to be accomplished to achieve 100% Completion. There are a few ways you can keep track of their progress in each of the required tasks.

The Link's below with open up a check list where you can find anything you are looking for, Ihave left a few different ones but they pretty much all do the same thing.
Interactive checklist[mapgenie.io]
Ign Checklist
100% Completion Wiki[reddead.fandom.com]
RDR1 100% Checklist Spreadsheet[docs.google.com]

Map [mapgenie.io]
Ign Map
Map Master[mapmaster.io]

Link to the original source where some of the info in this guide is from.
https://reddead.fandom.com/wiki/100%25_Completion_(RDR)
https://mapmaster.io/games/red-dead-redemption-1/maps/western-border-states
https://mapgenie.io/red-dead-redemption/maps/world
https://www.ign.com/maps/red-dead-redemption/world