Perfect Buck Pelt
How to get Perfect Buck Pelts for the Legend of the East Satchel in Arthur's story in Red Dead Redemption 2.
You will need a rifle (scope optional), the bow, or a rope.
Time of Day: sunrise to just after sunset.
Bucks can be found in many locations across the map, primarily in a sort of rectangular area that goes between the Dakota River (south of Valentine down to the mouth of the river), east to Emerald Ranch, and south-east to Rhodes.
One of the best areas I know of to hunt for them is south of Braithwaite Manor, in a glade between Old Harry Fen and Catfish Jacksons. (Marked on the map below with the player icon.)
Be aware that the bucks will charge and knock you down if they see you before you see them.
Bow
Charles teaches you how to hunt deer with the bow during the game's prologue, and the same technique is used when hunting the bucks. For those that have not played the prologue recently the basic refresher on that is that you want to move slowly as you approach the buck, stopping whenever they lift their heads. Also pay attention to the direction of the wind, and try to stay downwind of the buck. You can see what way the wind is blowing by entering Deadeye and looking for the smoky scent tendrils coming off of Arthur. Try to keep those pointed away from the deer, not toward them.
When you draw the bow you will begin to lose stamina, so only draw it when you are ready to shoot at the deer. Then hold it for a moment, because the draw will determine how far the arrow travels, and if you just let go as soon as you draw it back, then the arrow is only going to go a few yards from where you are.
Aim for the head or aim just behind the shoulder blade of the foreleg. Headshots have less risk of damaging the pelt.
Rifle
The rifle is a good way to get bucks, however, it will also send everything else running for the hills. It is also risky to use if you have a group of bounty hunters after you, since the sound of the gunshot will alert them to your location.
If you plan to hunt a lot, and do not want to have to sneak up each time, then getting a scope for your rifle and upgrading the distance and accuracy is a good idea.
Hunting with the rifle is as easy as finding the bucks, determining which (if any) is a three star deer, and targeting the head for the cleanest kill.
Rope
You can hunt pretty much anything you can lasso, but be aware that you will be losing stamina while holding the critter at the end of the rope. You want to get proficient with chasing the bucks down on horseback, lasso them, then hop off the horse close to the buck to get close enough to use your knife to kill the animal. This is one of the safest ways to assure that you are not going to damage the pelt, and the bucks can not get you if they try to charge when you are on your horse. (I know because if I see one charging, I hop up on my horse fast.)