First things first, install the VMod and it's required coremods in you world.

In your world, you'll need the custom crafting table. This is made by dropping an iron ingot and a log together.
Use one of the following recipes to make a spear:


To use the spear, left click to attack. Right click to throw.
Throwing a spear takes it out of your inventory. You'll need to pick it up wherever it landed.
You can use tipped arrows to have a tipped spear next throw.