What I do is trace original artwork onto tracing paper (or even oven baking paper is fine)
Then tape that to cardboard , go over the tracing with a pen to get an indentation on the cardboard , remove tracing paper , go over  indent with pencil/pen .
Then cut out shapes for 1 colour .

Rinse and repeat above to make stencil for second colour .
Also have alignment marks to be able to align correctly against cab .

Thats pretty much how i do it, except when i go over the tracings i have sheets of carbon paper under the tracing paper to transfer the tracings onto the cardboard.

Ktm450; In the past i have just cut through the tracing paper and into the cardboard (using separate tracings for each colour) but for this cab i traced BOTH colours onto one large sheet of tracing paper, (actually several sheets taped together).