[Closed] banksy style stencil grafitti, how do they make the stencils?

thick, good quality, cardboard and a craft knife.

Put an image into photoshop or similar and "fiddle" with it to make a sillouhette, print onto paper, stick paper to cardboard then use a craft knife (works best with thin card). simple images work better, and remember that lots of cutting out is a PITA! Spray lightly and from a reasonable distance else the stencil becomes soggy or paint will run and ruin the image.

You're both accomplices now!

We cut stencils for the complicated bits, but other bits can just be painted using the base stencil as a guide. But yes, it's just cardboard cut outs.
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You can also get some plasticised "card" that is a little less likely to go soggy if you're doing a lot of spraying at one time.