First of all you will need to get the Recurve Bow from a weapon locker or general store. You can either buy it or activate radio towers to get it for free. Also buy and equip the Markman Sight. It's a very useful attachment for long range shots. Now search for some enemies and put a marker on their location on the world map. Walk away until the marker shows a distance of 70+ meters and take the shot while the enemy is standing still. The Welshore Wrecker's House Outpost (coordinates X: 437.9, Y: 795.0) is near the starting area of the game and one of the best locations to get this done early.
 

Toxophilite Video Guide