*Mild spoilers* After you finish up Desmond's sequence on the Skyscraper, and start playing Sequence 6, you obtain the rope darts. Basically, they're a spear head on a rope. Once you're back out on the Homestead, you will see an Artisan mission for a woman named Myriam. Talk to her and you'll notice she's hurt, and you'll go back to the house with her. You then must go out and kill the poachers who hurt her, and Achilles even mentions how this is the perfect opportunity to use the rope dart. You will have 3 darts from this point, so it's best to buy 2 rope darts before going out to get the poachers.
Once you climb to the tree stand, you have to go out and kill the poachers. To do this specific achievement, be up in a tree near the poachers. Hold Y to grab one with your rope dart and pull back on the left stick to hang them from the tree. If you don't get enough poachers during the mission (there's 4-5 you'll be able to this on in the mission), you can just find some red coats wandering around in the Frontier to do this on too.
If you don't wish to have to go look for some red coats, you could also use the rope dart on the first two poachers (as they are the easiest to get) and reload the checkpoint for the mission. You'll be back up in the tree stand, and you can kill them again, reload one more time and get the achievement in the next go around. (Credit to Chicago AJ for this tip)