1. ATV DLC
   
2. Silver Ridge Peaks DLC

1. Equip Alexander Longbow
   
2. Buy a bunch of the 700 gr. Traditional Arrows
   
3. Ride around in the Saguache Plains on SRP looking for plains bison
   
4. Find herd and chase them down
   
5. Get in front of or next to the herd
   
6. Shoot the nearest bison, the easiest shots are front on but this can lead to getting stomped so its better to shoot from the side. Make sure to lead slightly as the bow has a minor delay and the arrows are slow. The arrows have insane penetration so you can hit the heart from basically any angle.
   
7. Get back on the ATV and chase down the other bison
   
8. Repeat steps 5 and 6

I've attached two images of successful heart shots to show where to aim, as well as one instance where I missed but shot through the skull and out the rear of the animal to show the fact that you can hit the heart from any angle.

Let me know if anything is confusing. I can try to make a video demonstrating this if needed. Good luck and happy hunting!