This is not my save, but it does work.
The ONLY thing left to observe is ONE more BLACKSMITH entry.
In order to get it to work you will need to do some Homestead missions, though. You will need to do the Homestead missions up until the point you get an additional blacksmith mission. In this mission you/Connor should be you chasing after an escaped soldier, and killing him before he escapes. You will know this was a Blacksmith mission when it adds a stable with a horse upon completion. After you do the blacksmith mission go to his shop/house and observe him doing anything besides sharpening the axe or hammering metal in his shop. Remember to observe him during day time, morning is best. Remember that in order to get this mission to appear in the Homestead you must complete the other Homestead missions available (approximately 4-5). Homestead missions are short so it shouldn't take too long.
NOTE: There is a Homestead mission started in Boston that you will have to complete as well.
Here is the link to the save:
Above all else, enjoy your 10 gamerscore. Stupid glitchy achievement.
CREDIT: David Do0M