This is the most common method. Almost all PvP-oriented resource packs come with a custom icons.png . It requires no mods and works on every server.
In vanilla Minecraft 1.8.9, the crosshair is not just a static pointer; it is an interactive debugging tool. crosshair minecraft 1.8.9
This is arguably the most lightweight and respected crosshair mod for 1.8.9. This is the most common method