This is my first how-to, so I hope it's helpful.
To start with, I've taken my pre-painted Predator tank. After some creative criticism, I realized it looked "brand new" without battle damage or dirt/mud. I decided to go all-out on this piece by creating realistic bullet holes.
What you need (very simple):
Pin vice (aka modeling/hobby drill)
Model(s)
Metallic paint
Take your model and decide where you want the battle damage. Keep in mind, it'll look like machinegun fire, so it will be sparse, misaligned, and sporadic.
Next, line the drillbit up with the model to imply the direction the bullets struck from. This is very important, as it'd look awkward if the bullets seemed to have come straight down onto the vehicle, or from below it.
Now, drill very lightly into the plastic. Make sure you drill into the model and not through it! If the drillbit happens to penetrate the other end, it's not a big deal, but try to avoid it.
After you've made up some bullet holes, you'll want to paint them with some metallic (or black/rust) color to show it's been damaged [I used boltgun metal]. Depending on where the vehicle was struck from, you may want to create "splash" damage or chipped paint around the holes, or possibly ricochet marks from the bullets that bounced off.
Repeat as necessary. Enjoy your $40 hunk of Swiss-cheesed plastic!
*EDIT*
I would like to note that the distance between bullet holes symbolizes the distance of the shooter, and you can use larger sized drillbits to indicate different types of ammo hits [my goal was to make autogun and small-arms fire].
Here's some photo examples of this tutorial:
Setup
Step 1: Drill holes
Step 2: Paint holes