For long-range "one-taps," try to stay completely still or crouch while dragging the fire button at a slower speed to prevent bullets from going over the head. Visual Indicator Hack (Yellow to Red)
For the best chance of landing "all red" shots, use these high-sensitivity settings tailored for the responsive touchscreens of modern iPhones:
Searching for "hacks" to get "all red" headshots in Free Fire
Given these points, I'll provide some general information on how to play Free Fire effectively and the risks associated with seeking or using hacks.