Here's how I went about putting the "freehand" on top of the Land Raider Redeemer I just finished for my latest commission.
The first step was to talk with the client and get a good idea of the kind of imagery he wanted on the top. After that, I sent him a few rough sketchs to make sure we were both on the same page with our ideas and to get his approval.
Then I redrew the rough sketch a little cleaner and to scale (there's only about 3 inches by 1 inch to work with). With that line drawing, I take a pencil and shade the back making cheap transfer paper.
I put the draiwng in place and use a ballpoint pen and trace the image leaving a faint pencil mark on the actual vehicle.
Then I take a small tip, permanent pen and redraw the image in place, making a corrections as needed.
My Project Link: Space Marines Commision
Additional FTW Painting links:
Decals: How to get that painted on look by Pit of the Oni
How to paint freehand icons by Ron Saikowski (FTW)