The following steps make it relatively simple
1) trace the outline of the door you wish to decorate onto some white paper (graph paper also works well for symmetrical designs)
2) draw the design on the paper, staying within the traced outline of the door
3) using the side of your pencil, rub graphite onto the back side of the paper behind the design
4) tape the sheet of paper onto a sheet of plasticard
5) trace over the design with a pencil, pressing down hard. This should transfer the design to the sheet of plasticard underneath.
6) remove the sheet of paper, and cut out the design with a sharp exacto knife.
7) glue the cut-out design to the vehicle door, and prime and paint as normal.
Take it slow on any curves in the design, as plasticard tends to cut/break better in straight lines than on curves. If you want to get fancy you can use a couple layers of overlapping designs.
Below are a pair of top rhino doors I created for a Deathwatch Killteam Rhino.