Hold it right there, outlaw! This article is littered with spoilers, so I reckon you might ought'a mosey on down the road if you don't want to read any plot details.
This article was a featured article for the site in November 2011.
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 accomplishment covered under the achievement/trophy entitled "Zed's Dead, Baby."
The Walkthrough, below, details everything that needs to be accomplished to achieve 100% Completion. There are a few ways the player can keep track of their progress in each of the required tasks:
The in-game "STATS" menu displays overall progress towards many of the goals, but without specific details.
The Rockstar Social Club (free account required) provides a detailed visual checklist that is automatically updated when the player connects their Social Club account to their console. While the list is a useful visual reference, it may lag slightly behind the in-game stats due to synching requirements. Saving your game and refreshing the Social Club page on your computer can typically re-sync the checklist.
There is a spreadsheet available here that can help the player keep track of their own progress. It uses information from this wiki and was built in Microsoft Office 2010.
Each of the nine required outfits that is unlocked counts as 0.5%, for a total of 4.5%. Only the outfits listed below are required for 100% completion.
Obtaining all thirteen safehouses is worth a grand total of 2%, meaning each safehouse is worth 2/13 of a percent, or roughly .1538%, however since the percentage is rounded to the nearest tenth, some will appear as being worth .1%, and some as .2%.
Note that the Safehouses must be purchased so that the icon changes from blue to green. A safehouse count can be quickly checked in Stats under "General," after the list of hideout completion times.
Each of the seven gang hideout that is cleared (at least once) counts as 0.5%, for a total of 3.5%. The PS3-exclusive hideout, Solomon's Folly, does not count towards 100% completion.
Each of the twenty locations at which a Bounty Target is successfully captured or killed is worth 0.1%, adding to a total of 2.0% towards 100% completion.
For 100% completion, eight bounty locations must be completed in New Austin, eight in Nuevo Paraiso and four in West Elizabeth. Note: It is possible to need 9 bounties in New Austin and only 7 in Nuevo Paraiso, plus the four in West Elizabeth. Bounty total for each location can be viewed under (score) in the players (stats) menu. After completing a location, new bounties will continue to be posted for the same location, but only one successful completion is needed/counted for 100% completion.
Bounties are placed at approximately 7:00 am and 5:00 pm in any of the major settlements, although some postings have been observed at other times. The player should be in the town or settlement where the bounty is to be posted (i.e. in Armadillo if going after a New Austin bounty) prior to the posting time. At this time, a lawman will come and post the wanted poster (this may take a moment, depending on how close the Safe House is).
In order for the bounty to count, the on-screen message about capturing the target dead or alive must be displayed. If sniping from far away, the bounty may not count.
Completion of 18 of the 19 available Stranger missions counts for 0.5% each, for a total of 9%. (The "I Know You" mission does not count toward 100% completion)
Each of the six minigames that the player wins counts as 0.5%, for a total of 3%.
For each minigame to be counted towards the 100% completion stat, the player must beat all players at the event (in line or at the table). For example, to complete Arm Wrestling the player must beat the person at the table as well as any people in line after him. Each game only needs to be won at a single location. For blackjack you have to leave the table with more chips than when you started, because you can't "win" blackjack.
All of the following minigames are needed to obtain an outfit. Combining the two tasks speeds up the overall process of acquiring 100% completion.
Three locations sometimes fail to register as "found" even after purchasing and using the appropriate maps: Repentance Rock, Tanner's Span and Mason's Bridge. The former is a cluster of steep rock formations in the middle of Rio Bravo (west-north-west of Fort Mercer); the latter two are railway bridges located to the southwest and southeast, respectively, of the Pacific Union Railroad Camp. Manually journeying to these locations (and crossing the bridges) will unlock the locations and count towards 100% completion rate.
There's also a strange glitch observed on Xbox 360 - two locations from Nuevo Paraiso are not actually found there, but they rather are found in West Elizabeth (they unlock after using Tall Trees map and the stats show the player with 94/94 overall, 47/46 in New Austin, 17/16 in West Elizabeth, but only 30/32 in Nuevo Paraiso).
To save time, effort and money in the minigames some of the players can be made to go away. In Arm Wrestling and Five Finger Fillet the people in the line can be knocked over (but they will stop playing the game if pushed or attacked) and will then wander away leaving only the man at the table. He will become more difficult to beat but the player will only have to beat him. Similarly, on a poker table if there is more than 1 person sitting (Normally 3-4 players is too much) and the player wants to reduce the amount of players, go near the table, take out a gun and aim towards any one person (make sure that he is facing the player and able to see that the player have taken aim on him) Do not shoot'!'. Once the player does this, all players will get up and run away. Now go to a nearby table like a blackjack table or a little bit far (preferably where an action is performed, like sitting down on a blackjack table or checking wares in one of stores). As soon as the player sits on the blackjack table for e.g., get up immediately and go back to the poker table. There will be 1 player or max 2 players at the table. If the player is playing poker, the player can get caught cheating, duel the player and they will be gone from the table. Do this to all but 1 player then face them alone. Note: The fewer players there are, the more likely the player is to be caught cheating. (This is most noticable when there is only one player left, so if the player is planning on cheating leave at least 2 players remaining).
Instead of going to all 94 locations, the player can simply purchase all of the available maps saving a lot of time.
The physical map that comes with the game reveals all 94 locations.
If only 31 out of 32 locations are showing as discovered in Nuevo Paraiso, try heading to The Old Bacchus Place. This location will sometimes show up as a missing location in Nuevo Paraiso, even though it is physically located in New Austin. Plata Grande is another commonly missed location (it's a loop between Tesoro Azul and Sidewinder Gulch).
In some cases a player may achieve 100% completion, but not get the achievement. Turning off the Xbox and reload the game may unlock the achievement automatically on the next game load.
After applying Title Update 1.04, some players were unable to fully complete the game due to a bug with locations and bounties not being tracked properly. This issue was fixed on 12/2/10 and should no longer be an issue. Full details of the problem and solution are available in this Rockstar support article.
The "On the Trail of de Vaca" achievement/trophy sometimes randomly unlocks after playing multiplayer on LAN, however this does not count for the 100% and all locations must still be discovered.