You should be able to get this fairly quickly. But if you are having trouble, complete a hunting trip in America on Rookie and you will unlock a scope. Once you have that, use the Bolt Rifle and when hunting, click on the Scope icon on the left to zoom. Zoom in on an animal's head, tap "Steady", then fire. Should only take a few tries to nail one in the head.
If you are still having trouble, just keep playing. After a few hunting trips, select "Profile" in the main menu, then select "Skills" and start upgrading your Accuracy. This will reduce the drift on the reticle.