This is just a general topic for how you customize toys. My contribution is how to create burn marks on a Gundam model kit (Gunpla).
First, you need the kit you are going to customize (it must be abs plastic), a Q-tip, and a (preferably) grey or black Gundam panel lining marker.
1. Apply the marker, I recommend a bold line for wherever you want the burn to be.
2. Rub the mark (while still wet) using the Q-tip in the direction in the line to smear it. The more you rub it, the more it will smooth and blend into the plastic.
3. Let it dry and you're done!