This is really simple to do and takes no special tools.
All you need to do is either imagine or simply draw a line down the center of the top hatch (A). Then draw a horizontal line (B) where you want the side points of the arrow head to be.
Then connect the points across the top. You can make the width of the arrow anything you want, but I just divided the distance from the middle to the sides of the hatch equally on each side and used that. It seemed to be the right width overall.
Paint and weather to your liking and you're all set.
UPDATE: Since this post is fairly old, I decided to go back and add a few more links to it to expand on some of the topics covered here.
When to paint freehand onto your models is important
Two ways to map out your freehand
Why it's important to start weathering with a clean model
My Project Link: Space Marines Commision